[codex] clean up plugin source layout

[hrz6976]

What changed

• move the plugin implementation and tests from repository root into src/
• update local and CI shared-library builds to target ./src
• remove unused parsed configuration and runtime state
• remove a redundant global pool mutex
• move settings reads and host logging out of scheduler-critical lock sections
• simplify degraded scheduler candidate bookkeeping
• avoid per-refresh regex and duration-table allocations
• trim unused cooldown decode fields and test wrappers
• document the repository layout

Why

The repository root mixed implementation files with distribution metadata, configuration examples, and release tooling. This refactor gives the plugin a clear source boundary while reducing dead code and incidental runtime work without changing its routing, quota, dashboard, management, or ABI behavior.

User impact

Runtime behavior and configuration remain unchanged. Contributors now work under src/; make, CI, packaging, and release outputs retain their existing interfaces and filenames.

Validation

• make test
• go test -race ./...
• go test -cover ./... (52.5% statement coverage)
• make package VERSION=0.1.1-dev
• release checksum and ZIP layout verification
• C ABI export verification
• maximum required glibc symbol remains GLIBC_2.34