docs: warn stable users about unsafe config migration

[Burakodbs]

Summary

• warn stable 4.19.4 users that the reasoning-unification migration can produce a configuration rejected by that release
• document the safe workaround while npm latest remains affected

Changes

• add a version-specific warning to the configuration migration reference
• explain that even --dry-run can resume a pending migration journal before dry-run handling
• cover automatic migration triggers during startup and install
• add a Known Issues entry with symptoms, backup guidance, workaround, and fix status

QA & Evidence

• What was tested: git diff --check
  Observed result: passed with no whitespace errors
  Artifact: commits 6fa9bdcb1, 3d3573db9, and 5d058bec8
  Why sufficient: the change is limited to two Markdown files
• What was tested: issue Doctor 4.19.4: false 'Unknown config key: agents.*.models' + premature deprecation warnings for fallback_models/variant that runtime still reads #6644 and PRs fix(omo-opencode): accept migrated agent model chains #6602 and fix(runtime-fallback): honor canonical model chains #6668
  Observed result: issue Doctor 4.19.4: false 'Unknown config key: agents.*.models' + premature deprecation warnings for fallback_models/variant that runtime still reads #6644 and PR fix(runtime-fallback): honor canonical model chains #6668 are open; PR fix(omo-opencode): accept migrated agent model chains #6602 is merged
  Artifact: linked below
  Why sufficient: distinguishes the merged validation support from the remaining runtime fallback work

Risks & Residuals

• Documentation-only change; no runtime behavior is modified.
• Full project checks were not run locally because bun install --frozen-lockfile exhausted the temporary filesystem while its prepare hook fetched unrelated frontend submodules. Temporary install artifacts were removed afterward.

Automated Checks

git diff --check

Related Issues

Related to #6644.
Validation support: #6602.
Runtime follow-up: #6668.

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