docs(design): make 056 aimdb join self-contained for a from-scratch implementation

[lxsaah]

Description

Revises design 056 (aimdb join CLI) so it no longer builds on the planning branch prototype, and can be implemented from scratch on main.

Removed: the "port join.rs from planning" starting point, the D1–D5 defect table, the "kept from planning" tests, and the planning commit references.

Restated: D1–D5 become plain requirements, R1–R7 (§1).

Newly specified (gaps a fresh implementation would otherwise have to decide):

• Profile content: the whole response is written, so fields the CLI doesn't know survive (043 §6). JSON null is dropped, since TOML has none. Required fields are listed.
• Redirects: none are followed. Otherwise an https:// base could redirect a 307/308 POST, token included, to http://.
• GitHub endpoints can be overridden only by an internal parameter used in tests, never by an environment variable or flag.
• Token hygiene: no token in Debug output or in error messages.
• Discovery: handling of 5xx, malformed JSON and network errors. --app behaviour after a 404.
• Profile writes: a no-clobber persist without --force, and an fsync of the directory.
• Exit codes: CliError::exit_code() plumbed into main, which currently always exits 1. Exit code 2 for "no usable auth kind" is now in the table.
• Dependencies: reqwest pinned to 0.12, since 0.13 renames the TLS features. tempfile moves to a normal dependency.
• Tests: an expanded test list (§7) and a new sequencing (§9).

Docs only, no code changes.

Related Issue

• n/a

Checklist

• I have read the CONTRIBUTING.md document.
• My code follows the project's coding standards.
• I have added tests to cover my changes. (docs only)
• All new and existing tests passed (make check). (docs only)
• I have updated the documentation accordingly.

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https://claude.ai/code/session_018WAwJGRt66jBy58M9hRtTr

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