feat(hooks): PreToolUse + PostToolUse subprocess hooks

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Summary

Extends ignis's subprocess hook system with two tool-level events — its founding motivation: programmable per-tool transformation/vetoing.

• PreToolUse runs before the permission gate. It can block the call (continue:false / exit 2 → the model gets Blocked by hook: <reason>) or rewrite the tool's args (updatedInput, parsed back to JSON). The permission gate and the tool both see the rewritten args.
• PostToolUse runs after the tool (success or error, via isError). It can rewrite the result the model sees (updatedOutput) or just observe. It cannot block (the tool already ran).
• Optional tool-name matcher per hook spec (bash, mcp__*, *); absent = all tools. New toolName/toolInput/toolResult/isError envelope fields.

Reuses the entire existing machinery: chained dispatch, soft-fail→Warning, Landlock sandbox + env allowlist, /hooks listing + reload. Any hook failure degrades to the original value + a Warning (never blocks, except an explicit PreToolUse block); a malformed updatedInput soft-fails to the prior args.

Integrated in the per-tool path: PreToolUse (sequential) → permission gate → tool.call → PostToolUse (inside execute_single_tool, so it works under parallel fan-out). Main-agent only — sub-agent tool calls are unhooked in v1. Engine-only — soft-failures surface via the Warning event the Ink frontend already renders.

Verification

• Unit: matcher globbing, config parse (matcher + both events), protocol round-trip of tool fields, HookEvent::ALL=4.
• Integration (real subprocess hooks): PreToolUse rewrite, exit-2 block, matcher-skip, PostToolUse rewrite.
• Full gate: fmt, clippy (clean), 825 Rust tests.
• Code-review pass: no significant findings (gate-after-rewrite, sub-agent isolation, parallel safety all confirmed).
• Dogfooded through the real binary + a live model: a PreToolUse hook rewrote the model's echo hello-original → echo REWRITTEN_BY_HOOK, and the rewritten command is what executed (tool block shows bash(echo REWRITTEN_BY_HOOK)).

Note for sequencing

Touches hooks/* + the per-tool path in agent/mod.rs. The mechanical ..Default::default() / tool: None test-literal churn is from adding fields to HooksConfig/HookSpec/DispatchContext.

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