feat: Implement no-tmux mode with beads as data plane (gt-vc3l4)

Enable Gas Town to operate without tmux by using beads for args transport:

- Add `attached_args` field to beads AttachmentFields
- gt sling: Store args in bead description, graceful fallback if no tmux
- gt prime: Display attached args prominently on startup
- gt mol status: Include attached_args in status output
- gt spawn --naked: Assign work via mail only, skip tmux session

Agents discover args via gt prime / bd show when starting manually.
Docs added explaining what works vs degraded behavior in no-tmux mode.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/no-tmux-mode.md

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+# No-Tmux Mode
+
+Gas Town can operate without tmux or the daemon, using beads as the universal data plane for passing args and context.
+
+## Background
+
+Tmux instability can crash workers. The daemon relies on tmux for:
+- Session management (creating/killing panes)
+- Nudging agents via SendKeys
+- Crash detection via pane-died hooks
+
+When tmux is unstable, the entire operation fails. No-tmux mode enables continued operation in degraded mode.
+
+## Key Insight: Beads Replace SendKeys
+
+In normal mode, `--args` are injected via tmux SendKeys. In no-tmux mode:
+- Args are stored in the pinned bead description (`attached_args` field)
+- `gt prime` reads and displays args from the pinned bead
+- No prompt injection needed - agents discover everything via `bd show`
+
+## Usage
+
+### Slinging Work with Args
+
+```bash
+# Normal mode: args injected via tmux + stored in bead
+gt sling gt-abc --args "patch release"
+
+# In no-tmux mode: nudge fails gracefully, but args are in the bead
+# Agent discovers args via gt prime when it starts
+```
+
+### Spawning without Tmux
+
+```bash
+# Use --naked to skip tmux session creation
+gt spawn gastown/Toast --issue gt-abc --naked
+
+# Output tells you how to start the agent manually:
+#   cd ~/gt/gastown/polecats/Toast
+#   claude
+```
+
+### Agent Discovery
+
+When an agent starts (manually or via IDE), the SessionStart hook runs `gt prime`, which:
+1. Detects the agent's role from cwd
+2. Finds pinned work
+3. Displays attached args prominently
+4. Shows current molecule step
+
+The agent sees:
+
+```
+## ATTACHED WORK DETECTED
+
+Pinned bead: gt-abc
+Attached molecule: gt-xyz
+Attached at: 2025-12-26T12:00:00Z
+
+ARGS (use these to guide execution):
+  patch release
+
+**Progress:** 0/5 steps complete
+```
+
+## What Works vs What's Degraded
+
+### What Still Works
+
+| Feature | How It Works |
+|---------|--------------|
+| Propulsion via pinned beads | Agents pick up work on startup |
+| Self-handoff | Agents can cycle themselves |
+| Patrol loops | Deacon, Witness, Refinery keep running |
+| Mail system | Beads-based, no tmux needed |
+| Args passing | Stored in bead description |
+| Work discovery | `gt prime` reads from bead |
+
+### What Is Degraded
+
+| Limitation | Impact |
+|------------|--------|
+| No interrupts | Cannot nudge busy agents mid-task |
+| Polling only | Agents must actively check inbox (no push) |
+| Await steps block | "Wait for human" steps require manual agent restart |
+| No crash detection | pane-died hooks unavailable |
+| Manual startup | Human must start each agent in separate terminal |
+
+### Workflow Implications
+
+- **Patrol agents** work fine (they poll as part of their loop)
+- **Task workers** need restart to pick up new work
+- Cannot redirect a busy worker to urgent task
+- Human must monitor and restart crashed agents
+
+## Commands Summary
+
+| Command | Purpose |
+|---------|---------|
+| `gt sling <bead> --args "..."` | Store args in bead, nudge gracefully |
+| `gt spawn --naked` | Assign work without tmux session |
+| `gt prime` | Display attached work + args on startup |
+| `gt mol status` | Show current work status including args |
+| `bd show <bead>` | View raw bead with attached_args field |
+
+## Implementation Details
+
+Args are stored in the bead description as a `key: value` field:
+
+```
+attached_molecule: gt-xyz
+attached_at: 2025-12-26T12:00:00Z
+attached_args: patch release
+```
+
+The `beads.AttachmentFields` struct includes:
+- `AttachedMolecule` - the work molecule ID
+- `AttachedAt` - timestamp when attached
+- `AttachedArgs` - natural language instructions
+
+These are parsed by `beads.ParseAttachmentFields()` and formatted by `beads.FormatAttachmentFields()`.