feat(watch): add --exec hook to run a command on attention transitions

[jongio]

Adds dispatch watch --exec <cmd>, an automation hook that runs a command every time a session changes attention state while streaming. This turns watch into a building block for notifiers, webhooks, and desktop alerts without polling the JSON output yourself.

What it does

• --exec <cmd> runs the command through your shell on each attention transition (waiting, working, thinking, gone, and so on).
• Session context is passed as environment variables, never interpolated into the command string, so untrusted session metadata cannot change what runs:
  • DISPATCH_SESSION_ID, DISPATCH_SESSION_STATE, DISPATCH_SESSION_PREV_STATE (none for a newly seen session), DISPATCH_SESSION_REPO, DISPATCH_SESSION_BRANCH, DISPATCH_SESSION_FOLDER, DISPATCH_SESSION_SUMMARY.
• Hook output and non-zero exits are written to stderr; a slow or failing hook never stops the watch loop, keeping stdout clean for --json consumers.
• --exec requires streaming mode and is rejected when combined with --once. It composes with the existing --repo, --branch, and --folder filters.

Example

dispatch watch --exec '[ "$DISPATCH_SESSION_STATE" = waiting ] && \
  curl -s -X POST "$WEBHOOK_URL" -d "session=$DISPATCH_SESSION_ID needs input"'

Implementation notes

• New package seam watchExecFn (default runWatchHook) lets tests capture invocations and env payloads without spawning shells.
• hookShell() picks the shell per OS ($SHELL -c / /bin/sh -c on Unix, %COMSPEC% /c / cmd.exe /c on Windows).
• hookEnv() builds the DISPATCH_SESSION_* slice and is unit tested directly.

Verification

go build ./..., go vet ./..., go test ./... -count=1, golangci-lint run --timeout 10m (0 issues), and mage deadcode (OK) all pass.

Closes #329