matrix: config loading now merges optional TOML with CLI/env/secret inputs

[l5yth]

Config loading now merges optional TOML with CLI/env/secret inputs, adds container-aware defaults, and ships a new clap-based CLI so the bridge can be configured dynamically without hard‑coding Config.toml. I also updated container entrypoint defaults, docs, and the Rust toolchain to 1.92.

• Added a clap CLI with config/state/override flags and wired it into bootstrap (matrix/src/cli.rs, matrix/src/main.rs)
• Reworked configuration loading to merge TOML + CLI/env + secret files, added container detection/defaults, and expanded unit tests for precedence and container behavior (matrix/src/config.rs)
• Updated container entrypoint to set container awareness + secrets dir defaults without forcing a config path (matrix/docker-entrypoint.sh)
• Documented precedence, env vars, secret files, and container defaults in matrix/README.md
• Bumped the builder image to Rust 1.92 and regenerated the lockfile (matrix/Dockerfile, matrix/Cargo.lock)
• Replaces matrix: add docker env boilerplate #605

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Codecov Report

❌ Patch coverage is 95.82505% with 21 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
matrix/src/config.rs	95.82%	21 Missing ⚠️

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