feat(agent): summarize spawn-only text outputs

[ymote]

Summary

• add auto_summarize as an opt-in spawn_only manifest flag and carry it through plugin loading into ToolRegistry
• generate a bounded profile-LLM summary for opted-in spawn_only completions with workspace text file outputs over 4KB
• append the summary as a background follow-up with a read_file path hint while preserving the existing produced-files fallback
• add manifest, loader, symlink-safety, threshold, and background execution coverage
• harden the existing CLI-agent executable test helper against Linux ETXTBSY so required CI can rerun reliably

Validation

• cargo test -p octos-agent --test spawn_only_handle_envelope -- --nocapture
• cargo test -p octos-agent auto_summar --lib -- --nocapture
• cargo test -p octos-agent --tests --no-run
• cargo clippy -p octos-agent --tests -- -D warnings
• cargo test -p octos-cli cli_agent_adapter::tests::argv_config_does_not_invoke_shell_metacharacters --lib -- --nocapture
• cargo fmt --check
• git diff --check

Notes

• bundled skill manifest flips for deep-search/news remain out of this PR, matching the issue note that manifest updates are a separate follow-up

Closes #897

CI / merge status

• GitHub CI is green on refreshed head f6df6fe3dc3570f8d9cb54a5dbc9c4f401dd0839: check, check-matrix, dashboard, swarm-app, test-octos-agent (lib), test-octos-agent (integration), test-octos-cli, typos, and author-email passed. Optional expansion jobs were skipped by workflow policy.
• Merge remains branch-protection blocked by required review (BLOCKED, REVIEW_REQUIRED).

[ymote]

Refresh/validation evidence for current-main head f6df6fe3dc3570f8d9cb54a5dbc9c4f401dd0839 (base 4e58cd4972c82d9916b1d05cb044782c6c106e7b).

Local environment: isolated checkout /private/tmp/octos-ws1-audit-527a, CARGO_TARGET_DIR=/private/tmp/octos-1266-current-4e58-target, CARGO_INCREMENTAL=0.

Local validation passed:

• cargo fmt --all
• cargo fmt --all -- --check
• git diff --check origin/main...HEAD
• git diff --check
• git ls-files --others --exclude-standard (empty)
• cargo test -p octos-agent --test spawn_only_handle_envelope -- --nocapture (3 passed)
• cargo test -p octos-agent auto_summar --lib -- --nocapture (5 passed)
• cargo test -p octos-agent --tests --no-run
• cargo clippy -p octos-agent --tests -- -D warnings
• cargo test -p octos-cli cli_agent_adapter::tests --lib -- --nocapture (8 passed)
• cargo clippy -p octos-cli --tests --no-deps -- -D warnings

GitHub validation passed:

• CI run 26569779007: check, check-matrix, dashboard, swarm-app, test-octos-agent (integration), test-octos-agent (lib), test-octos-cli, and typos all passed.
• Author check run 26569779025: reject blocked author/committer emails passed.

Result: #897 closure implementation is refreshed on current origin/main and green. The PR is mergeable at the git level, but branch protection reports mergeStateStatus=BLOCKED and reviewDecision=REVIEW_REQUIRED, so I did not merge.