feat(cli): agent-native provisioning with init --agent and agents claim

[wondercreatemaster]

Summary

• closes feat(cli): agent-native provisioning — vouch init --agent plus a claim handshake #606: vouch init --agent --agent-caller <name> provisions an agent-scoped KB, stamps agent.caller as the persistent proposer identity, and writes a local credential (chmod 0600) that is never echoed to stdout
• emits a claim token / vouch agents claim <token> command (plus --json for skills); claiming from a project KB transfers ownership and adopts knowledge through the same review gate as vouch adopt via additive adopt_kb (personal-fallback adopt stays byte-stable), leaving the agent credential and agent-KB artifacts untouched; also registers into the feat(cli): agent registry — list, pause, resume, revoke, and per-agent audit replay #607 project agent registry when possible
• mcp/jsonl/cli fall back to the stamped agent.caller when VOUCH_AGENT is unset
• also fixes capture.load_config to use coerce_numeric so malformed numeric capture settings degrade to defaults (unblocks ruff + the existing regression test on test)

Test plan

• python -m pytest tests/test_agent_provision.py tests/test_adopt.py -q
• python -m ruff check src tests
• python -m mypy src
• diff-cover coverage.xml --compare-branch origin/test --include 'src/vouch/*' --fail-under 100 (100%)
• full pytest tests/ -q --ignore=tests/embeddings green locally
• CI green on this PR
• manual: vouch init --agent --agent-caller demo --json → credential path exists, stdout has no credential value → cd project && vouch init && vouch agents claim <token> binds and adopts

[plind-junior]

closing on CI, not on approach.

test (py3.12) and test (py3.13) are both failing on the current head. this repo's test branch requires all three matrix jobs plus build sdist + wheel green before anything merges, so this cannot land in its present state, and a red test matrix is not something a reviewer should be reading around.

the scope concern from my earlier pass still stands and is worth acting on when you re-file. this is +1572 across 9 files and bundles four separable things: a credential file written to disk, a claim-token flow, a KB ownership transfer, and a new persistent proposer identity — plus an +83/-34 rewrite of adopt.py, a load-bearing module, with no matching change to tests/test_adopt.py. ownership transfer is the highest-risk path here and it currently rides entirely on test_agent_provision.py.

split it and the cheap half lands immediately: --agent-caller stamping a persistent proposer identity is a small, well-contained change to attribution. the credential + claim-token + ownership-transfer half is a security surface and deserves its own diff, its own tests, and its own review.

also, house convention: please drop the "Made with Cursor" trailer from the description. this repo does not carry AI-tool attribution on contributions.

reopen once the matrix is green and the adopt changes have tests — the underlying idea in #606 is fine and i am not rejecting it.