docs: rewrite AGENTS.md for the post-Go, Rust-only codebase

[KaluJo]

Summary

AGENTS.md was written during the Go → Rust port and still reads like a transitional document — it tells contributors "the Go implementation under ../cli/ is the behavioral source of truth", points at cli/cmd/ and cli/internal/ directories that no longer exist, and frames everything in terms of "parity" with a build that was deleted in the rust-port merge.

This PR rewrites it as a Rust-only contributor guide and folds in the conventions that have hardened since the port:

• Crate-shape diagram for pcr-core / pcr-cli / pcr-napi, pointing at NPM-INTERNALS.md for the napi rationale (load Rust addon into node.exe to avoid AppLocker).
• "On-disk format stability" replaces "Parity with the Go CLI" — same byte-compatibility constraints (auth.json, projects.json, drafts.db schema, content_hash, exit codes), but reframed as "don't break what's already on disk" rather than "match Go".
• New "TUI conventions" section: screens/ vs widgets/ layout, theme tokens, the HeaderBar { user: auth::load()... } pattern (so we stop hard-coding user: None like in show.rs / help.rs), modal cancel / confirm rules, and the narrow-column Wrap { trim: false } rule we hit while fixing the empty-bundles hint.
• New "Source watchers" section documenting the main-thread / non-blocking invariant and the path_norm::canonicalize_project_path_cache requirement.
• "Running locally" now lists the exact lint commands CI runs (cargo fmt --check, cargo clippy -- -D warnings) so the next contributor doesn't tag a release that dies in lint (as just happened with v0.2.6 → re-tag).
• Pointer to RELEASING.md for the full release flow.

No code changes; this is a contributor-facing rewrite only.

Test plan

• Read through the rendered file on GitHub — the link to NPM-INTERNALS.md works, the table renders, all referenced files (tui/screens/show.rs, auth.rs, tui/screens/status.rs, path_norm, golden.rs, etc.) actually exist at those paths.
• No remaining mentions of cli/cmd/, cli/internal/, "the Go implementation", or "during the transition".

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