INF-1484/fix: stop swallowing pre-commit hook failures

[brtkwr]

Context

Spotted on workflow-portal#120: prettier failed inside the action's log ("files were modified by this hook") but the pre-commit check reported success. The INF-1484 python-modules refactor (#21) replaced the old set +e exit-code capture with:

python -m pre_commit_action.run_hooks ...
echo "EXIT_CODE=$?" >> "$GITHUB_OUTPUT"

Composite shell: bash runs with -e, so a hook failure aborts the step before EXIT_CODE is written; continue-on-error: true masks the step failure; and the report step then runs exit '', which is exit 0. Every consumer's pre-commit check has reported green regardless of hook failures since #21 merged.

Changes

• Capture the exit code with set +e (as the pre-INF-1484/feat: extract inline shell to Python modules + add pytest #21 shell did), so EXIT_CODE is always written.
• Drop continue-on-error: true - the step can no longer fail spuriously, and keeping it would mask real infra errors (e.g. uvx unavailable).
• Report step: exit ${{ steps.pre-commit.outputs.EXIT_CODE || '1' }} - fail loud if the output is ever missing instead of silently succeeding.
• Sticky comment status now derives from the exit code in render_comment.py (the step outcome is always success now that the exit code is captured) - the PRE_COMMIT_OUTCOME env is gone. Tests updated.

Validation

• run_hooks.py already propagates the subprocess return code via sys.exit (covered by pytest); the bug was purely in the yaml wiring.
• After merge, workflow-portal#120's pre-commit re-run should go red on the prettier finding (currently false-green) - that's the expected behaviour change.

Ticket

• https://linear.app/tillit/issue/INF-1484

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[gemini-code-assist]

Code Review

This pull request modifies action.yaml to capture the exit code of the pre-commit run by disabling immediate exit on error (set +e) and removing continue-on-error: true. It also ensures the final status reporting step fails loud if the exit code is missing. However, a critical issue was identified: because the last command in the pre-commit step (echo) always succeeds, the step's outcome will always be evaluated as 'success'. This causes the subsequent step to mistakenly post a "Pre-commit success" comment on the PR even when hooks fail. A fix was suggested to determine the outcome using the captured EXIT_CODE instead of the step outcome.

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