Registry 0.6.0: status-returning handlers, typed params, single exit mapping

[samifouad]

What changed

Implements the 0.6.0 contract from #22 (rfd#61, approved):

• Handlers return a status. CommandSpec/SubcommandSpec handlers are now fn(&Context) -> Result<ExitStatus, CommandError>. CommandError::Usage exits 2, CommandError::Runtime exits 1 (GNU convention), Ok(ExitStatus) carries the code (ExitStatus::SUCCESS in almost all cases, ExitStatus::from_code for commands whose contract promises a code).
• One exit mapper. Registry::run(args) -> ExitCode parses, dispatches, prints every error in one format, and is the only place that maps to a process exit code — main becomes fn main() -> ExitCode { registry.run(std::env::args()) }. Consumers with special no-argument behavior (help with exit 0) check for an empty argv before calling run. The underlying dispatch() now propagates the handler's HandlerResult.
• Typed params. Context::flag(), Context::param::<T>(), Context::param_or(), Context::param_list(); a bad value is a usage error in one format (invalid value for --port: …). ParamSpec gains kind: ParamKind::Value (last wins) / ParamKind::List (--tag a --tag b accumulates in order). Flag and param keys canonicalize to the spec name during parsing (-v reads as --verbose).
• In-process command tests. Handlers write through Context::out()/Context::err(); Registry::run_captured(argv) returns (ExitCode, stdout, stderr) without spawning a process.
• Docs say Deka, not Tana; README documents the new contract with a wire-up example.

Version bumped to 0.6.0; after merge this needs a v0.6.0 tag (release.yml publishes and verifies — tagging is Sami's).

Evidence

• cargo test --all-features → 10 suites, 0 failures (47 lib tests including 6 new contract tests: exit-code mapping, captured output, parse-error format with suggestions, typed-param validation, list accumulation, canonical flag access)
• cargo clippy --all-features --all-targets → 0 warnings
• Revert proof: flattening the exit-code mapping to always-1 fails run_maps_handler_results_to_exit_codes_in_one_place (left: ExitCode(1), right: ExitCode(2))
• Consumer tests (registry_bridge, context_env, context_bridge, context_extensions) migrated to the new signatures and pass; alias canonicalization is asserted in registry_bridge
• Note: the repo's timing-sensitive update tests (health probe exceeded 2s, token temp-name race — the latter already filed as token_file: concurrent write_token races on temp name (flaky test on main) #24) flake intermittently on this loaded machine, on main as well; the full suite passed clean on the final run

closes #22
-kimi