refactor(pipeline): extract pipelineschema package and shared schema cache

[Viktor (tiulpin)]

Summary

Split out of #226 to keep that PR focused on migrate. Pulls the pipeline JSON-schema helpers out of internal/cmd/pipeline into a reusable internal/pipelineschema package and the on-disk schema cache into cmdutil.FetchOrCachePipelineSchema, so both pipeline and the upcoming migrate command share one definition. Adds config.ConfigDir() as the single source for the config-dir path, and makes pipeline validate fall back to the embedded schema when run unauthenticated instead of erroring.

Changes

• New internal/pipelineschema package: Bytes, Compile, ValidateWithSchema, ConvertYAMLToJSON, HostedAgentNames.
• New cmdutil.FetchOrCachePipelineSchema (24h on-disk cache, embedded fallback, corrupt-cache self-heal); removes the duplicate cache logic from pipeline/schema.go.
• config.ConfigDir() extracted; update state path and schema cache now derive from it.
• pipeline validate validates against the embedded schema when not authenticated (warns instead of failing).
• Bump fus-reporting-api-go.

Design Decisions

Straightforward.

Example

$ teamcity pipeline validate .tc.yml   # no auth
⚠ Not authenticated - validating against the embedded schema; run 'teamcity auth login' to use your server's schema
✓ .tc.yml is valid

Test Plan

• Unit tests pass (just unit)
• Linter passes (just lint)
• Acceptance tests pass (just acceptance) — N/A — no command/flag surface change
• If adding a new command/flag: added .txtar test in acceptance/testdata/ — N/A — no new command/flag
• If adding a data-producing command: includes --json support — N/A
• If modifying --json output: no field removals/renames (additive only) — N/A
• If changing docs-visible behavior: updated docs/, skills/, and README.md — N/A — internal refactor + unauth fallback only
• External contributors: links a status:finalized issue — N/A — internal change