feat: add shared Context extensions (RFD 61)

[samifouad]

Dispatchers can now attach consumer-owned state to Context before handing it to command and subcommand handlers. Extensions stores TypeId -> Arc<dyn Any + Send + Sync> and exposes insert<T>() and get<T>() -> Option<&T> through extensions() / extensions_mut().

Context retains Clone, Debug, Send, and Sync. Clones copy the map and share values through cheap Arc clones, so stored types need neither Clone nor Debug. Replacing an entry affects only that context's map. The borrowed getter avoids an Arc clone for handler reads. README and API docs describe these semantics; consumers using struct literals must migrate to Context::new(args) and assign env as needed.

Validation and consolidated QA/seam review:

• All five extension tests pass: roundtrip/replacement, type/context isolation, command/subcommand handoff, Send + Sync, and clone pointer identity/shared mutation/local replacement. The existing args/cwd bridge test also passes.
• cargo check --locked --target wasm32-unknown-unknown --no-default-features --features registry passes.
• cargo clippy --locked --all-targets -- -D warnings passes; changed Rust files pass rustfmt checks and git diff --check passes.
• cargo test --locked was run but is not green: the unchanged concurrent token-write test fails with temporary-file collisions. Reproduced independently on origin/main (ead2d1a) with cargo test --locked --test token_file_hardening. Parallel runs also encountered existing update health-probe timeouts; the initially failing probe passes in isolation.
• cargo test --locked -- --test-threads=1 --skip concurrent_writes_leave_a_complete_private_token_and_no_temp_files passes. No unrelated token-file or timeout code is changed.

Refs dekaruntime/rfd#61.

-codex