docs: document doctor session-recovery diagnostic (#4539)

[Hmbown]

Summary

Adds one clarification paragraph to docs/GUIDE.md documenting the doctor session-recovery diagnostic added in #4539: read-only, filename-level comparison, six statuses, additive to doctor / doctor --json, explicit CODEWHALE_HOME suppresses ambient inspection.

Produced by the Claude issue worker on #4542 as the final end-to-end verification of #4537/#4543: exact-@claude maintainer gate, signed branch from main (b6a9ab0, signature verified), no auto-PR. This PR is the human-click step.

Closes #4542.

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[claude]

Claude finished @Hmbown's task in 43s —— View job

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Review: docs: document doctor session-recovery diagnostic (#4539)

• Gather context (diff, CLAUDE.md, AGENTS.md)
• Review docs/GUIDE.md changes for accuracy
• Verify against actual doctor implementation from fix(doctor): diagnose recoverable legacy sessions #4539
• Post final review

Verdict: accurate, no changes needed.

I cross-checked the added paragraph in docs/GUIDE.md:123-130 against the actual DoctorSessionRecoveryStatus implementation in crates/tui/src/main.rs:

• Six statuses: isolated, no_legacy_sessions, migration_pending, migration_incomplete, migration_complete, scan_failed — matches the enum at crates/tui/src/main.rs:4524-4530 exactly, including the as_str() string forms at 4536-4541.
• Read-only, filename-level comparison: matches doctor_session_recovery_report (crates/tui/src/main.rs:4633-4736), which only calls read_dir/symlink_metadata on .json filenames and never opens file contents — confirmed by its own doc comment at 4626-4632.
• Additive to doctor / doctor --json: confirmed — print_doctor_session_recovery_report (4800) and doctor_session_recovery_json (4895) are both called alongside the existing legacy-state report rather than replacing it.
• Explicit CODEWHALE_HOME suppresses inspection: confirmed — codewhale_home_is_explicit short-circuits to Isolated before any directory scan (4652-4655).
• The "same legacy-path migration described above" cross-reference correctly points back to the ~/.deepseek → ~/.codewhale migration text a few lines earlier in the same doc.

No correctness, provider-safety, reuse, test-coverage, or security concerns — this is a pure documentation addition (+9/-0) with no code changes, and every technical claim in the new paragraph is verifiably true against the implementation it documents.