refac now defaults to the Claude Messages API (model claude-opus-4-8); OpenAI
remains available via `provider = "openai"`. New `src/anthropic.rs` talks to the
REST API with reqwest (no official Rust SDK): x-api-key + anthropic-version
headers, top-level `system`, and required `max_tokens`. It adapts refac's flat
message list to Anthropic's shape — lifts the system prompt out of messages and
merges the consecutive user turns to satisfy user/assistant alternation — and
marks the static system prompt + few-shot examples `cache_control: ephemeral`
so repeat calls only pay for the varying input. Config gains `provider`, optional
`model` (defaulted per provider), and `max_tokens`; secrets hold either/both keys.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

…intln

Per review: drop the ad-hoc REFAC_DEBUG env-gated eprintln and use the existing
tracing setup, gated by the subscriber's level filter like the rest of the code.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Per review: `provider` is now optional. When it isn't set explicitly (config
file or REFAC_PROVIDER), resolve it from which keys are configured — only an
OpenAI key -> OpenAI; Anthropic-only, both, or neither -> lean Anthropic. An
explicit choice still wins. Adds resolve_provider() + tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Per review: a `max_tokens` config knob is a representable invalid state —
`provider = "openai"` + `max_tokens` does nothing (the OpenAI path ignores it).
Remove it. The Messages API still requires `max_tokens`, so hardcode it as a
constant in the anthropic module. Can be reintroduced later with real support
in both backends.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Per review:
- REFAC_PROVIDER no longer silently falls back to Anthropic on an unrecognized
  value — it errors with the accepted options.
- OpenAI default model o1 -> gpt-5.5 (current flagship).
- Drop a WHAT doc-comment on model().

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Per review: `refac login` now accepts `--provider anthropic|openai`; without it,
the user selects a provider via a dialoguer prompt rather than the key-inference
heuristic (you're choosing which key to add). Provider derives clap::ValueEnum.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Per review: the prompt-cache breakpoint was inferred from message structure
(last assistant turn), which baked in refac's usage pattern. Take the static
prefix length from the caller instead — refactor() passes chat_prefix().len(),
the part that's fixed across calls. build_request places the breakpoint at that
boundary and never groups a varying turn into the cached prefix. Also corrected
stale comments (alternation is no longer required; merging is just grouping).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Per review: that claim could go stale; omit it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Per review:
- module doc was tmi / drifting — cut to a one-liner.
- removed the verbose MAX_TOKENS comment (kept the const; the API requires the field).
- replaced the stringly-typed content/cache `type` tags with enums (BlockType, CacheType).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

The match arms say it; the doc comment carries the why.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Per review. Also drop the WHAT comments restating the test assertions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Per review: drop the mirrored label array; map choices through Debug instead.
Also drop a comment per suggestion.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Message is now `{ role: Role, fields: Vec<String>, cache: bool }` instead of a
single-string OpenAI-shaped struct. A turn carries one or more text fields (a
transform user turn is [selected, transform]); `cache` marks the last turn of a
static prefix.

- anthropic: each field -> a content block; empty fields render as `(empty)`
  (the API rejects empty text); `cache` -> cache_control on the turn's last
  block. Drops the old consecutive-same-role merge and the cache_prefix_len
  parameter — the data model carries both now.
- openai: its own `OpenAiMessage` wire type; the adapter joins a turn's fields
  and drops `cache`.
- prompt: chat_prefix marks its last message cached.

Live-tested against Anthropic: a normal transform and an empty-selected
generate ("find .") both work. 5 unit tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

~$2.00 of Opus 4.8 output at $25/M. Verified the API accepts it for the model.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Per review:
- CacheControl and the content block are internally-tagged unions; use
  `#[serde(tag = "type")]` enums instead of a struct with a manual `type` field.
- message roles are `Role` enums (anthropic ChatMessage and OpenAiMessage), not
  strings; Role gains a lowercase serde repr and `as_str` goes away.
- the response block is a tagged enum (`Text { text } | Other`) rather than a
  stringly `kind` compared against "text".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

api.rs is now just the provider-agnostic core (`Message`, `Role`). OpenAI's wire
types and `complete` move to a new openai.rs; the Anthropic-only
`field_or_placeholder` moves into anthropic.rs (and OpenAI no longer applies it
— it tolerates empty content). Also drops the unused OpenAI edits-API types
(EditRequest/EditResponse/Choice), which were dead.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Drop the removed `max_tokens` config setting, note provider is inferred from
keys when unset, fix the API-key link to Anthropic (the default), and reword
"still supported" -> "also supported".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Review-loop findings:
- the saved secrets file held the API key but was created world-readable
  (default 0644); create/force it 0600 on unix.
- LogEntry.provider was a Debug-formatted string; use the Provider enum so the
  log matches the config-file casing and stays typed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>