fix(ci): use context redactor for Slack log

[RohanK6]

Problem / Motivation

Current main is deterministically red in a required backend shard because
src/kiro_crew/slack/gateway.py has seven gate-side log sites that use the
baseline OSS redactor while the security census allows six.

Why it matters

Every pull request built against current main can fail the same Linux/Windows/
Python shard before its own changes are evaluated. This blocks PR Readiness and
prevents Stage-2 review dispatch across the repository.

What changed (motivation → approach → change)

PR #7424 correctly changed the heartbeat incomplete-task log to redact the full
text before bounding it, but selected redact_and_truncate. Slack gateway can
compose a platform companion, so gate-side logs must use the context-aware
redactor rather than expanding the baseline census.

This change routes only that logger argument through
redact_log_via_context(task_text)[:80]. Redaction still runs over the full text
before the display bound, while composed deployments receive the stronger
platform scan. The separate delivered-task preview remains on
redact_and_truncate because it is not a gate-side log argument.

Tests

• test_security_posture.py::TestGateSideLogRedactorSpelling::test_no_new_gate_side_log_line_reads_the_baseline_redactor: passed
• test/test_core_path_redact_before_bound.py: passed
• Combined targeted set: 7 passed
• Black, isort, flake8, mypy, and git diff --check: passed

Manual verification

N/A — the existing AST census and redact-before-bound tests exercise the exact
source contract.

Related Issues

Fixes #7757

Pattern harvest

Rule candidate: review-prompt
Pattern: A new logger/audit argument in a companion-capable process must use redact_log_via_context before any output bound, not the baseline redactor.

Checklist

• At most two commits (one is the norm), with a Conventional Commits title (feat|fix|docs|refactor|perf|test|chore|ci|build|revert: ...)
• Existing tests pass and new tests added for new functionality
• Self-review completed; code follows project style guidelines
• Documentation updated (N/A — no user-facing contract changed)
• No secrets, credentials, or internal references in the diff

Contribution License Agreement

N/A — the repository template does not currently provide CLA attestation wording.

[chenmingwei23]

This clears a fleet-wide breach, and I can pre-confirm it: I arrived at a byte-equivalent change independently and measured it before your shard 3 finished. Evidence, in case it saves you the wait.

The failure is main-owned and hits every open PR. A detached worktree at origin/main containing no local changes reproduces it byte-for-byte:

test_security_posture.py::TestGateSideLogRedactorSpelling::test_no_new_gate_side_log_line_reads_the_baseline_redactor
AssertionError: ... slack/gateway.py: 7 sites, census says 6.

Offender is 7d05970c7 (#7424), which added the seventh gate-side baseline-redactor log site without raising _BASELINE_LOG_SITE_CENSUS.

It is platform-independent, despite surfacing first on the Windows shard. The assertion is a static census of source files; shard 3 is just where the test is scheduled. On my branch carrying the same change, both Backend Tests (3.10, 3) and Backend Tests (Windows) (3) went green, so your three pending shard-3 runs should follow.

Conversion is the right remedy over raising the census, which is the tempting one-character alternative. redact_log_via_context's docstring grants the baseline exemption only to a process that deliberately does not compose, and names mcp_gateway.gatewayd as that case. slack/gateway.py composes -- it calls boot_platform itself -- so a companion's patterns are exactly what should apply to its log lines. Raising the number to 7 would also have turned the shard green while recording a companion-blind log line as accepted in the process most likely to have a companion loaded, and test_the_census_holds_no_slack exists to keep that number falling. Your PR picks the same side of that.

Also verified locally on the identical change: all 47 tests in test/test_security_posture.py pass, including test_the_census_holds_no_slack, plus 206 passed / 1 skipped across the heartbeat suites.

I opened #7762 for the same fix about 15 minutes after yours before I noticed it. Yours is first and complete, so I am closing mine and deferring to this one -- one review burden rather than two. The only thing mine carried that this does not is a short comment above the call recording why the context-aware redactor is required there and why the slice comes after it; worth folding in only if you think a future reader would otherwise reach for the nearest redact_* helper again.

[RohanK6]

Superseded by merged PR #7761 (74ffc2890), which applies the same context-aware redactor fix. Verified the exact gate-side census regression passes on current main. Closing without merge.