Add portable record export across surfaces

[tony]

Summary

Add the initial portable export contract across CLI, TUI, and MCP using one deterministic, frontend-neutral renderer.

• Add agentgrep export for stable NDJSON or Markdown, bodies on by default, standard output or explicit files, bounded selection, no-clobber output, and explicit --force.
• Add HUD /export [PATH] and /export-thread [PATH] for one selected record or its observed thread, with private collision-free defaults and all identity, rendering, and disk work off the Textual message pump.
• Add a read-only export_records MCP tool for one to 20 existing search refs, bodies opt-in, a 400 KiB inline artifact ceiling, bounded opaque refs, and no server-side filesystem authority.
• Share one exact allowlist that excludes source paths, origins, titles, session identifiers, and arbitrary metadata while preserving canonical IDs and an artifact-level schema version.
• Harden source-alias protection, atomic writes, phase-error privacy, legacy ref compatibility, duplicate selection handling, PATH_MAX ref round-trips, and docs/runtime schema parity.

Relationship

This PR targets the branch for #111 because deterministic record identities are its prerequisite. It is independent of the bookmark PR and can be retargeted to master after #111 lands.

Closes #81

Tested

• Full repository gate: Ruff, formatting, ty, 2,891 passed, 3 skipped, 3 expected failures, and Sphinx.
• Format × body × cardinality × selection permutations; CLI stdout/file/force/refusal; MCP size/ref/privacy boundaries; TUI races, reset, teardown, large payloads, and static non-blocking guards.
• Live Textual pane: private and explicit Markdown writes, literal bracketed filenames, no-overwrite refusal, preserved results, private modes, unchanged source stores, responsive narrow layout, and clean exit.

Goal

Idempotent completion condition for /goal. It asserts an end state, not an action: if the state already holds, the first evaluation passes and nothing is edited.

/goal PR #122 is green on its base and its ADR stops claiming a number. Work ONLY in the worktree agentgrep-issue-81-export; check out no branch in the shared master tree; never push. Report FIRST: git rev-parse --abbrev-ref HEAD (prints issue-81-export; judge local HEAD, not a remote ref), git -c rerere.enabled=false merge-tree --write-tree --name-only issue-80-deterministic-identities HEAD, NO_COLOR=1 AGENTGREP_TUI_WATCHDOG=1 PATH=$PWD/.venv/bin:$PATH env -u VIRTUAL_ENV -u UV_NO_SYNC ./.venv/bin/python -m pytest -m '' --reruns 0 -q, uv run ruff check ., uv run ty check, and grep -A3 '^## Status' docs/dev/adr/*portable-record-export.md. End state: merge-tree prints zero CONFLICT lines against that LOCAL base tip as it stands, the other three exit 0, and that Status line records the ADR's NUMBER as provisional pending merge-time allocation. If that tip moved (#111 may rebase it), rebase onto it once rather than waiting on or reverting anything. Those three pass today, so the Status line is the work; renumber and reslug nothing, numbering being a merge-time call. Without the PATH prefix the shell resolves agentgrep to the master checkout, which lacks the export verb, so doc examples fail. Non-goals: no rebase onto master, no PR retarget, no landing #111. Preserve ADR 0011: identity, rendering, scanning, path validation and disk writes stay off the pump, with no scandir, lstat, open, sqlite, subprocess, network, json load/dump or whole-artifact CPU reachable from a pump callable a hop down (the hook sees neither, so trace by hand), pump entrypoints @pump_only, workers @offload. Keep schema_version and the format value extensible; call no shipped format set, ceiling or allowlist permanent; let export deny no index or provider. Or stop after 12 turns and report what remains.

• Proof — git rev-parse --abbrev-ref HEAD && git -c rerere.enabled=false merge-tree --write-tree --name-only issue-80-deterministic-identities HEAD && grep -A3 '^## Status' docs/dev/adr/*portable-record-export.md && NO_COLOR=1 AGENTGREP_TUI_WATCHDOG=1 PATH=$PWD/.venv/bin:$PATH env -u VIRTUAL_ENV -u UV_NO_SYNC ./.venv/bin/python -m pytest -m '' --reruns 0 -q
• Idempotence — Once the Status line carries the provisional-number note the proof is purely read-only: the branch still merges clean into its local base tip and the suite still passes, so a re-run changes nothing; a base tip moved by Expose deterministic IDs and tighten TUI controls #111 costs exactly one rebase and then the same steady state.
• Blocked by — nothing (landing Expose deterministic IDs and tighten TUI controls #111 is an explicit non-goal; the branch is judged against its own local base tip as it currently stands)
• Blocks — nothing
• Preserves — The export format value, inline ceiling and field allowlist stay versioned and extensible so csv/html/mermaid and vendor messages profiles remain addable; export makes no claim that search reads only live stores, leaving Durable prompt corpus and derived search indexes (ADR 0019) #142's corpus and Persistent SQLite DB index with cache-aware search #47's index provider open; ADR 0011 placement of identity, rendering, scandir/lstat and disk writes off the pump is retained; the ADR number stays available to the other branch claiming it.