feat(ci): add soft budget warnings to xtask ci plan

[EffortlessSteven]

Summary

Builds on PR L (BudgetPolicy + load_budget_policy). The planner now computes a Vec<BudgetWarning> and emits GitHub workflow commands (::warning::, ::error::, ::notice::) so PR Plan annotates the run when estimated LEM exceeds the configured thresholds.

This is PR O in the multi-PR CI roadmap. Stacked on PR L (#3761) → K (#3758); will retarget main as the stack lands.

Tiering

Configurable via policy/ci-budget.toml. Default thresholds:

Estimated LEM	Level	Notes
≤ 25 (preferred_default_lem)	none	normal
26–35 (default_limit_lem)	notice	"above preferred default"
36–75 (elevated_limit_lem)	warning	"review touched-area mapping"
76–125 (hard_limit_lem)	warning	"is high; review lane breadth"
> 125	error	references the override labels

The error level is downgraded to warning when the PR carries the full-ci or ci-budget-override label.

Informational only

PR O does not block merges:

• run() still returns Ok(()) regardless of warning level.
• The PR Plan workflow already has continue-on-error: true — even ::error:: does not fail the job.
• Hard enforcement (failing the status check) is a deliberate future change, gated on observed actuals from PR N (ci-actuals.yml).

Schema additions to ci-plan.json

"warnings": [
  { "level": "warning", "message": "Estimated CI spend 56 LEM is high (> 75); review whether this PR genuinely needs this lane breadth." }
]

Step summary now includes a "Budget warnings" section with icons (ℹ️ / ⚠️ / 🚨).

Tests (5 new, 19 total passing)

Test	Asserts
warnings_empty_for_pennies_band	empty PR → no warnings
warnings_notice_above_preferred_default	Cargo.lock-only PR crosses 35 → warning tier
warnings_error_at_hard_ceiling_without_override	full-ci + kernel diff crosses 125 with override → warning
warnings_emit_error_when_over_ceiling_without_label	stricter policy fixture; error tier without override
ci_budget_override_label_downgrades_error_to_warning	ci-budget-override label downgrades the error

cargo test --locked --no-default-features -p xtask --bin xtask ci_plan
# test result: ok. 19 passed; 0 failed

Out of scope

• PR P: ripr advisory → soft-gate promotion (consumes lane.stage taxonomy from PR K).
• Hard budget enforcement (status check) — future PR after a few weeks of actuals.
• Aggregating actuals across runs into a durable dashboard.

https://claude.ai/code/session_01S2yTnEYJcA3G2CyZn9bY4v

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