fix(terminal): terminal.session fails honestly instead of minting a false receipt

[gnanirahulnutakki]

Summary

Fixes #354. terminal.session was a silent no-op that still minted a permitted: true receipt — false success plus false signed audit evidence.

Root cause

crates/terminal/src/tools.rs TerminalSessionTool::invoke: after ensure_authorized, it read session_id/action, never touched command, and returned {"status":"ok"} with receipt("terminal.session", …), whose receipt(...) helper sets "permitted": true. No session was created, no command executed, nothing closed — yet the signed receipt chain recorded a permitted action that never happened.

The persistent-session backend does not exist; the tool was a stub that faked success.

Fix (tightest scope)

• Added ToolError::NotImplemented(String) to crates/tool-registry/src/error.rs — the tool-independent "registered but backend unimplemented" class.
• terminal.session now returns Err(NotImplemented) after the capability check (so an unauthorized caller still gets CapabilityDenied, not a leak of the not-implemented state).

Why this removes the misleading receipt: the runtime mints a tool receipt only on the Ok path. In fused-runtime/src/runtime.rs, a tool Err releases the cost reservation, fires the error lifecycle, and returns without pushing a ToolCallReceipt or minting — so an honest error produces no fabricated permitted receipt. map_tool_error already has a catch-all arm and there are no exhaustive ToolError matches, so the new variant is low-ripple. terminal.exec (which executes for real) is untouched.

Tests

• terminal_session_fails_honestly_and_mints_no_receipt — asserts Err(NotImplemented); fails against the old Ok{status:ok} + receipt behavior.
• terminal_session_denies_unauthorized_before_not_implemented — pins the cap-check ordering.

Verification (local)

• cargo test -p ardur-terminal -p ardur-tool-registry — pass (incl. 2 new regressions)
• cargo test -p ardur-e2e-tests — pass
• cargo clippy -p ardur-terminal -p ardur-tool-registry --all-targets -- -D warnings — clean
• cargo fmt --check — clean

Not merged — left for peer review.

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