docs(roadmap): add 12 peer-informed feature gaps

[emal-avala]

Summary

Extends ROADMAP.md with 12 concrete capability gaps surfaced by studying the current terminal-coding-agent landscape. Edits are additive only — no existing entries were removed, reordered, or re-scoped.

New / extended entries

Phase 1 — Documentation

• §1.10 Compatibility Matrix — living feature-comparison doc under Reference
• §1.11 Public SDK Documentation — `agent_code::sdk` facade, `examples/embed-agent` crate, semver contract

Phase 4 — Providers

• §4.3 Reasoning-Effort Controls — per-mode thinking/reasoning budgets (low/medium/high/none), `--effort` flag, `/effort` command, `/cost` integration

Phase 7 — Advanced

• §7.21 Phased desktop/web client delivery — hardened `--serve`, thin desktop shell over WebSocket, static web client, localhost-default security
• §7.22 Reversible Edit History — per-session edit journal, `/undo` and `/redo` grouped by turn
• §7.23 Multimodal Input in TUI — clipboard paste, drag-drop, `/image`, graceful degradation for text-only providers
• §7.24 Configuration Profiles — named `{model, sandbox, permissions, effort, tools}` bundles with inheritance
• §7.25 Enterprise TLS and Proxy Support — custom CA bundle, proxy env vars, `/doctor` reporting
• §7.26 SQLite Session Store — FTS5-backed `/history search`, migration tool, retention policy
• §7.27 `@`-Mention File Picker — inline fuzzy finder that pre-reads selected files into context
• §7.28 Turn-Lifecycle Hooks — standardized event surface (`turn_start`, `tool_call_end`, …) with JSON payload on stdin
• §7.29 Project Rules — `.agent-code/rules/*.md` glob-activated steering notes

Deliberately excluded

Items confirmed to already be implemented or already tracked in the roadmap: LSP integration, `web_search` tool, `/doctor`, `/review`, read-only plan mode, formatter event routing.

Test plan

• Render ROADMAP.md on GitHub and visually scan each new subsection for formatting
• Confirm `git diff --stat` shows additions only (1 file changed, 122 insertions, 1 deletion — the deletion is the `Last updated` date)
• Grep for each of the 12 new headings to confirm single occurrences

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