feat(skills): observability-planner for release-readiness pillar

## Skill Name

observability-planner

## Programming Language

Other — documentation-driven playbook skill (no executable scripts, same as `performance-slo-planner`)

## Purpose

The `release-readiness-gate` skill assesses an **Observability** pillar (logs, metrics, traces, alerting), but no dedicated planner produces a consumable artifact for it, so the gate falls back to codebase signals only. The repo has adjacent support — the `telemetry-foundations` skill (vocabulary) and `telemetry-overlay` instructions — but nothing that emits an `observability-plans/` artifact. This leaves Observability assessed less rigorously than the six pillars that do have producers. A standalone, `user-invocable` playbook skill (mirroring `performance-slo-planner`) would produce a first-class plan the gate can consume.

## Prerequisites

None — documentation-driven playbook, no external tools or scripts.

## Requirements

- Standalone `user-invocable` playbook skill (Procedure + Output Format), same shape as `performance-slo-planner`; no phase/state machine or agent.
- Reuse `telemetry-foundations` as its vocabulary reference rather than re-embedding telemetry semantics.
- Produce a plan covering: signals to observe per critical journey, metric/trace/log coverage, alerting + on-call wiring, dashboards, and observability gaps/backlog.
- Write output to `.copilot-tracking/observability-plans/-observability-plan.md`; add that folder to the disclaimer / untrusted-content applyTo globs (as done for `release-readiness/`).
- Register in the `project-planning` collection (planners live there; the gate consumes cross-collection).
- Update `release-readiness-gate` Default Pillars "Evidence source" to point at this skill.

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_Context: release-readiness-gate has two pillars without a dedicated producer (Observability, Operational Readiness)._

Start by reading the adjacent performance-slo-planner, telemetry-foundations, telemetry-overlay instructions, and release-readiness-gate. Add the user-invocable procedure and output format, register it in the project-planning collection, add observability-plans/ to the disclaimer globs, and update the gate's Observability evidence source; done means the dated plan artifact and all integrations are specified.

Written by the indexing model from the issue text.