docs: establish baseline glossary and ADRs

[Cannon07]

Summary

Baseline domain vocabulary and architectural decisions, established via a grilling session ahead of the bash-to-Lua migration in #47 phase 3. Pure documentation — no behaviour change.

• CONTEXT.md — 17 terms covering the full diff-preview pipeline: agent / integration, proposal / preview, RPC / socket discovery / pidfile, hook entry / core handler / dispatcher, change / status / origin prefix / indicator, tier 1 / tier 2, source/file/display path, visible-only mode / review gate, layout / renderer / reveal, hook context query, headless worker / in-process vs headless Lua.
• docs/adr/0001-origin-prefixed-status-values.md — Bash proposals are indicator-only, with origin-prefixed statuses. Records both the agent-behaviour observation (GPT-class models in Codex route edits through Bash) and the concurrency-safe prefix convention.
• docs/adr/0002-default-force-review-gate.md — Default to forcing the Claude Code review gate; opt-out via defer_claude_permissions.
• docs/adr/0003-inline-renderer-as-future-default.md — The inline renderer is the strategic direction; side-by-side reaches feature-floor parity then closes for further additions.
• docs/adr/0004-config-lives-only-in-neovim.md — Config lives only in the running Neovim; the bash layer queries on demand via hook context query.
• AGENTS.md + docs/agents/ — pointers so future AI agent sessions discover the canonical issue tracker, triage labels, and domain doc layout.

Why now

#47 phase 3 (port core-pre-tool.sh → core-pre-tool.lua) will reshape multiple modules. Doing it on a foundation of settled vocabulary makes the diff easier to review and the discussions easier to ground.

Follow-ups

Refactor opportunities surfaced while preparing the baseline, tracked separately so they don't pollute either this PR or the phase 3 work:

• Refactor: clean up path parameter naming in diff.show_diff #55 — Path parameter rename in show_diff (source path / file path / display path)
• Port character-level diff highlights to the side-by-side renderer #56 — Char-level highlight parity for the side-by-side renderer
• Decouple neo-tree concerns from diff.lua (mark_change_and_reveal) #57 — Decouple neo-tree concerns from diff.lua (mark_change_and_reveal)
• Unify integration installer interface (is_installed / install_state) #58 — Unify integration installer interface (is_installed / install_state)
• Remove deprecated ClaudePreview* command aliases #59 — Remove deprecated ClaudePreview* command aliases
• Rename /tmp/claude-diff-* tempfiles to /tmp/code-preview-* #60 — Rename /tmp/claude-diff-* tempfiles to /tmp/code-preview-*
• Split lua/code-preview/diff.lua into focused modules #61 — Split lua/code-preview/diff.lua into focused modules

Test plan

• No code changed — verify CI still passes (docs-only diff).
• CONTEXT.md cross-links resolve.
• ADRs link correctly to CONTEXT.md and to each other where referenced.

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