Add local engine profiling workflow

[tony]

Summary

• Adds dormant, privacy-safe engine profiling spans around discovery, planning, collection, source-level decisions, and subprocess calls.
• Adds scripts/profile_engine.py component runs for prompt search, conversation search, grep-shaped searches, and prompt-source enumeration, with Rich terminal output by default and explicit JSON/NDJSON machine formats.
• Extends benchmark artifacts with schema markers, sanitized profile payload capture, Rich nested-span rendering, a profile-engine command group, benchmark.py analyze for saved artifacts, rejection of empty --commands selectors, and load-time validation that profile-engine benchmarks request JSON output.
• Adds a Cursor IDE profile-engine benchmark set (profile-engine-cursor-ide) exercising the SQLite state.vscdb read path.
• Fixes broad searches dropping SQLite-backed stores during binary root prefiltering, so Cursor IDE matches reliably reach parser collection.
• Updates AGENTS.md, repo agent skills, and developer docs for the local profiling and benchmark-analysis workflow, and records the deliverables in CHANGES.

Refs #42. CI artifact upload remains separate in #43.

Test Plan

• rm -rf docs/_build; uv run ruff check . --fix --show-fixes; uv run ruff format .; uv run ty check; uv run py.test --reruns 0 -vvv; just build-docs;

[tony]

Code review

Found 1 issue:

1. _safe_profile_attribute_dict filters span attributes by key substring ("path" in denied_key_parts), which silently drops agentgrep_path_kind from benchmark.py analyze reports and Rich span tables. The value is a safe classifier literal (history_file / session_file / sqlite_db / store_file), not a filesystem path, and direct profile_engine.py artifacts retain it — so benchmark analysis loses the store-type dimension that the source-level spans were added to provide. Consider exact-key denial or an explicit allowance for agentgrep_path_kind.

agentgrep/scripts/benchmark.py

Lines 645 to 651 in 1afd722

	safe: dict[str, object] = {}
	denied_key_parts = ("argv", "command", "path", "query")
	for key, value in sorted(attributes.items()):
	if not isinstance(key, str):
	continue
	if any(part in key.casefold() for part in denied_key_parts):
	continue

Emit site for the dropped attribute:

agentgrep/src/agentgrep/__init__.py

Lines 3687 to 3689 in 1afd722

	"agentgrep_adapter_id": source.adapter_id,
	"agentgrep_path_kind": source.path_kind,
	"agentgrep_source_kind": source.source_kind,

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