refactor: replace blanket clippy::restriction with a curated workspace lint policy

[jhamill34]

Summary

Phase 1 of #1 (scoped in detail in that issue's discussion). Nearly every crate opened with #[warn(clippy::restriction, clippy::pedantic)] plus a long, drifted #[allow(...)] list. Clippy's own docs say restriction isn't meant to be enabled wholesale — and research turned up something sharper than that: three of the "commonly allowed" lints (match_ref_pats, needless_borrowed_reference, blanket_clippy_restriction_lints) are actually default-on via clippy::all, so allowing them wasn't opting out of a restriction lint at all — it was suppressing mainstream, warn-by-default clippy output.

• Centralized the policy in root Cargo.toml's new [workspace.lints.clippy] table. pedantic stays fully on. Every restriction-tier lint was reviewed individually:
  • Kept on: ref_patterns, map_err_ignore, allow_attributes_without_reason.
  • Re-enabled despite not being restriction-tier (were being suppressed anyway): too_many_lines, match_ref_pats, needless_borrowed_reference.
  • Explicitly allowed, each with a documented reason: implicit_return, question_mark_used, shadow_reuse/shadow_unrelated/shadow_same, single_call_fn, absolute_paths, mod_module_files/self_named_module_files, min_ident_chars, separated_literal_suffix, std_instead_of_core/std_instead_of_alloc (this workspace has zero #[no_std] crates), arbitrary_source_item_ordering (would force alphabetical ordering over the codebase's logical grouping for no readability gain), doc_paragraphs_missing_punctuation.
• Every hand-written crate now just does [lints]\nworkspace = true — the 14 that had the old per-crate block, plus auth/oauth_flow and prototypes/workflow_engine, which never opted into any lint policy at all before this.
• Removed service_loader's three #[cfg(test)] mod test { #[allow(clippy::restriction, clippy::pedantic)] } blocks — meaningless now that the crate root doesn't blanket-enable either group.
• Fixed the small, mechanical anti-patterns the old suppressions were hiding:
  • extern crate alloc; use alloc::sync::Arc (etc.) in 13 files across 9 ordinary std crates → plain std:: paths (zero crates here are #[no_std]).
  • 4 .map_err(|_| ...) sites that silently discarded the original error → now captured in the resulting error message.

Explicitly out of scope (tracked as follow-ups)

The policy change surfaces real fallout that's deliberately not fixed in this PR, to keep it reviewable:

• Phase 2 of #1: rewrite ~86 pre-2018 ref-pattern sites to match ergonomics #85 — the ~86-site pre-2018 ref-pattern rewrite this now flags as warnings (ref_patterns/match_ref_pats/needless_borrowed_reference), split into 3 stacked PRs by area.
• Phase 3 of #1: measure and fix clippy::too_many_lines fallout #86 — too_many_lines fallout once it's genuinely warn-on workspace-wide (overlaps with apicli: engine.rs is an oversized file mixing gRPC client logic, a JSON-Schema model, and merge/schemaify algorithms #12's apicli/engine.rs split).
• Phase 4 of #1: numeric-safety audit of api_caller's as-casts #87 — numeric-safety audit of api_caller's as-casts (kept allowed here with a documented reason pointing at this issue, not rushed into a lint-hygiene sweep).

Fixes #1

Test plan

• cargo build --workspace --all-features — clean.
• cargo clippy --workspace --all-features — zero errors; 193 warnings, all expected fallout (the ~86 ref-pattern sites plus scattered pedantic nits in the two newly-included crates), zero from any lint we deliberately allowed.
• cargo test --workspace --all-features — every suite passes, zero failures.
• cargo fmt --all -- --check — clean.

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