Sync coding-agent instructions with architecture

[FuJacob]

Summary

Refresh the tracked architecture roadmap and align coding-agent guidance with the product that currently ships: three generation backends, configurable acceptance, fail-open input interception, strategy-based insertion, explicit native runtime serialization, and the documented secure-field acquisition limitation.

Make AGENTS.md the single shared instruction source and reduce .claude/CLAUDE.md to Claude Code's supported @../AGENTS.md import. This removes the duplicated instruction copies that had drifted on backend, runtime, logging, and workflow details.

Validation

• git diff --check — passed
• Checked every Markdown link in ARCHITECTURE.md against the branch — no broken targets
• Verified .claude/CLAUDE.md resolves @../AGENTS.md
• Xcode build not run: documentation-only change with no source, project, dependency, or build-setting changes

Linked issues

None.

Risk / rollout notes

Documentation-only. The private gitignored .internal/ study guides are not included, and no unrelated worktree changes are part of this branch.

[FuJacob]

Superseded by #797 and the follow-on repository refactors (#798–#801). The remaining diff now points several architecture links back to pre-organization paths (for example Cotabby/Models/SuggestionSettingsModel.swift, Cotabby/Support/SuggestionAvailabilityEvaluator.swift, and Cotabby/Services/UI/OverlayController.swift), so merging it would make the current documentation stale. The accurate instruction and architecture updates are already on main.