[EPIC] action.yml hardening audit (F2 round)

gh issue create --title "[EPIC] action.yml hardening audit (F2 round)" --label "epic,audit" --body "## EPIC: action.yml hardening audit (F2 round)

Source: F2 audit round, 2026-09-03. This EPIC aggregates the 6 findings from the action.yml audit (2 HIGH, 2 MEDIUM, 2 LOW).

action.yml is the public contract of the action; HIGH-2 (missing outputs:) breaks the schema and the data-flow contract with downstream steps.

### HIGH

- [ ] #164 concurrency_lock description documents the v2.8.0 inline-lftp shape (existing; F2 added detail)
- [ ] #197 action.yml H2 — declare outputs (log_file etc.); entrypoint.sh writes to GITHUB_OUTPUT

### MEDIUM

- [ ] #198 action.yml M1 — concurrency_lock_poll_interval description doesn't document '> 0' restriction
- [ ] #199 action.yml M2 — top-level description says 'delete remote folder before' which is misleading

### LOW

- [ ] #200 action.yml L1 — mirror_verbose description promises '(0-3)' but validation only checks >= 0
- [ ] #201 action.yml L2 — server description is too terse for an input with complex URL shape

## Suggested resolution order

1. Land H2 (outputs:) in the same v2.11.3 as lib.sh CRIT-1 — declaring outputs is a prerequisite for the retry-classifier work that will write a 'last_error' output.
2. Land the documentation drift fix for #164 together with the entrypoint.sh C1 fix.
3. Batch the MEDIUMs + LOWs into a docs-cleanup release.

## Source

F2 audit 2026-09-03 (action.yml audit). Reporter: subagent-pekodbj7."