🐛 Make the Dependencies readiness check advisory instead of blocking

[monkut]

Problem

askcc develop exited 1 when an issue body lacked a ## Dependencies section, even when the issue had no dependencies.

_has_dependencies_section() is a heading regex (## Dependencies|Prerequisites|Context|Blockers) that never inspects the section's contents. The prepare and plan prompts instruct agents to always emit the heading — writing "None identified." when there are none — so the check gated on whether an agent had processed the issue, not on development readiness. Hand-written issues that skipped prepare/plan hit an exit 1 whose message ("No dependencies/context section found") read as "you have unlisted dependencies".

Change

The dependencies check still runs and still appears in the report — it just no longer affects an exit code.

• CheckResult gains advisory: bool = False (askcc/functions.py:306); the dependencies check sets advisory=True.
• _checks_passed() in askcc/cli.py ignores advisory checks. Both gates use it: the validate exit code and the develop pre-flight.
• _check_status() renders a failing advisory check as WARN; the report tallies blocking checks only.

Issue with acceptance criteria + assignee, no ## Dependencies:

Validation Report: <url>
------------------------------------------------------------
  [PASS] Acceptance criteria: Clear acceptance criteria found
  [WARN] Dependencies identified: No dependencies/context section found
  [PASS] Assignee confirmed: Assigned to: dev1
  [PASS] No blocking labels: No blocking labels found
------------------------------------------------------------
Result: PASS (3/3 blocking checks passed)

Docs corrected

The plan agent prompt (askcc/definitions.py:173) and the bundled handle-github-issue skill both asserted "develop rejects the issue otherwise" for the Dependencies heading. Now scoped to acceptance criteria and assignee, with the heading described as advisory but still expected from a planned issue. SKILL.md labels each check blocking/advisory and its sample report shows WARN. README rows for validate and the gating mechanisms updated.

Test plan

• uv run pytest — 284 passed (3 new)
• uv run poe check — ruff clean
• test_dependencies_check_is_advisory — only the dependencies check carries advisory=True
• test_validate_exits_zero_when_only_dependencies_missing — exit 0, [WARN] line, 3/3 blocking checks passed
• test_develop_proceeds_when_only_dependencies_missing — runner invoked despite the failing advisory check
• Existing test_validate_exits_one_on_fail and test_develop_fails_on_validation_failure still fail as expected on blocking checks

Version bumped to 0.2.15.

[monkut]

increment uv.lock with appropriate version