feat: add gt bead create/update/dep/list/search with prefix routing

[mk-imagine]

Summary

Add 5 new subcommands under gt bead that wrap bd commands with prefix-based routing via routes.jsonl:

• gt bead create — routes by --prefix flag, captures stdout for bead ID relay
• gt bead update <id> — routes by bead ID prefix
• gt bead dep <subcmd> <id> — routes by bead ID, skips subcommand token
• gt bead list — no routing (CWD context), pass-through
• gt bead search — no routing (CWD context), pass-through

Why: bd doesn't support Gas Town's routes.jsonl prefix routing. Agents calling bd create from the wrong CWD write beads to an invisible local database. gt bead create resolves the prefix to the correct rig database before executing bd. This makes bead operations deterministic regardless of the agent's CWD.

Pattern: Follows existing gt show and gt close routing pattern — resolveBeadDir() + chdir + strip BEADS_DIR + exec bd.

Implementation Steps

6 steps, single phase:

Step	Command	Routing?	Pattern
1	list	No	syscall.Exec pass-through
2	search	No	syscall.Exec pass-through
3	update	Yes (bead ID)	syscall.Exec + routing
4	dep	Yes (bead ID after subcmd)	syscall.Exec + routing
5	create	Yes (--prefix)	exec.Command (captures stdout)
6	Help text	N/A	Update beadCmd.Long

Steps 1+2 and 3+4 can be implemented in parallel.

Key Files

• internal/cmd/bead.go — register new subcommands
• internal/cmd/bead_{list,search,update,dep}_unix.go — new, platform-specific exec
• internal/cmd/bead_routing_test.go — new, unit tests for routing logic

Test plan

• go build ./... passes
• go test ./internal/cmd/... -run TestBead passes
• golangci-lint run clean
• Manual: gt bead create --prefix <pfx> --title "test" from town root creates bead in correct rig database
• Manual: gt bead update <pfx>-xxx --notes "test" from town root updates bead in correct rig database