feat(agent-context): compact command index by default; compact usage errors

[jexp]

Summary

• `agent-context` now emits a flat `path` + `short` index by default (~18 KB vs ~465 KB). `agent-context ` returns one command's details; `--full` keeps the old recursive tree. `schema_version` is now 2.
• Unknown flags/commands print the error, a one-line usage and a `Run ' --help' for details.` hint instead of the full help text and examples (exit code still 2).
• Rewrote the MCP agent-context test for the index shape, regenerated the skill bundle, added changie entries and updated `.agents/agent-context.md`.

Closes LABS-227

Test plan

• `make test`, `make fmt-check`
• New tests: index size cap, command detail, unknown path / `--full` conflict, compact usage errors
• `make lint`: only 2 SA1019 warnings in `internal/commands/query/coerce_test.go` (untouched file; local golangci-lint is 2.14.0 vs pinned 2.11.4), CI is the real check

🤖 Generated with Claude Code

[oplane-bot]

Oplane Security Review

Oplane reviewed this PR and did not identify any security requirements that need attention. Review the generated threat model.

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