feat: blast radius preflight gate via charter blast (#20)

[stackbilt-admin]

Summary

• Closes feat: blast radius preflight gate via charter blast #20 — adds a deterministic autonomous-safety gate that refuses to execute auto_safe tasks targeting architectural hubs.
• Runs charter blast --format json on file paths extracted from the task prompt; classifies severity on a 4-level ladder; blocks execution when critical.
• High/critical severities also inject a ## Blast Radius Warning section into the mission brief so the agent understands scope even when the task is allowed to proceed.
• Zero runtime dependencies beyond charter CLI 0.10.0 being on PATH. Gracefully no-ops when charter is unavailable.
• Applied symmetrically to taskrunner.sh and plugin/taskrunner.sh.

Severity ladder

Affected files	Severity	Behavior
0–4	low	silent
5–19	medium	silent
20–49	high	warning injected into mission brief
50+	critical	warning injected; auto_safe tasks refused

Environment knobs

CC_DISABLE_BLAST=1        # opt out entirely
CC_BLAST_WARN=20          # high threshold (default)
CC_BLAST_BLOCK=50         # critical threshold (default)
CC_BLAST_TIMEOUT=60       # charter blast timeout in seconds (default)

Graceful degradation

All of these are no-ops that let the task run as before:

• Charter CLI not installed
• CC_DISABLE_BLAST=1 set
• No recognizable file paths in the prompt (.ts / .tsx / .js / .jsx / .mjs / .cjs)
• charter blast fails or times out
• Target repo doesn't exist on this runner

How the gate fires

# Before
task { authority: auto_safe, prompt: "Refactor src/kernel/dispatch.ts ..." }
  → runs Claude, burns ~30 turns, produces 72-file PR, reviewer cries

# After
task { authority: auto_safe, prompt: "Refactor src/kernel/dispatch.ts ..." }
  → compute_blast_radius extracts dispatch.ts
  → charter blast reports 72 affected files (critical)
  → gate logs "⚠ GATE: blast radius critical — refusing auto_safe execution"
  → update_task_status("failed", "TASK_BLOCKED: blast_radius_critical — 72 files affected")
  → returns without spawning Claude

Operator can still force execution by changing the task's authority to operator and re-queuing. The gate only refuses auto_safe.

Real-world validation

Tested against a 263-file TypeScript Cloudflare Worker codebase:

Seed file	Affected	Severity	Gate action
version.ts (leaf)	5	medium	proceed
chat-session.ts (isolated DO)	1	low	proceed
dispatch.ts (orchestrator)	72	critical	refuse
types.ts (central types)	127	critical	refuse

Also tested:

• Prompt with no file paths → no-op, no blast data returned
• Prompt with .ts file that doesn't exist in repo → no-op, file filtered out
• High severity (20–49) → warning rendered, not blocked
• Low severity (0–4) → warning empty, not blocked

Seed count cap

The helper caps seed files at 10 to prevent runaway prompts from exploding the blast call. If a prompt lists 47 .ts files, only the first 10 (after sort) are fed to charter.

Prompt brief excerpt

When a high or critical severity fires, the mission brief looks like:

# MISSION BRIEF — Autonomous Task

You are operating autonomously in an unattended Claude Code session.
Read files before modifying them. Be thorough.

## Task
Refactor the dispatcher to use the new executor interface

## Blast Radius Warning
- Severity: **CRITICAL** — 72 files affected
- Seed files: src/kernel/dispatch.ts
- One or more seeds are in the top 20 most-imported files (architectural hub)
- Treat this as CROSS_CUTTING: review carefully before merging

## Project Context (auto-generated)
...

Test plan

• Bash syntax validated via bash -n on both taskrunner.sh and plugin/taskrunner.sh
• compute_blast_radius verified across low / medium / high / critical scenarios
• render_blast_warning verified for low (empty) and critical (full section) cases
• No-op path verified (no files in prompt)
• Charter CLI 0.10.0 with blast command on PATH confirmed working
• Gate logic (authority=auto_safe + critical) simulated in isolation

Related

• Depends on: @stackbilt/cli >= 0.10.0
• Complements: fix: auto-stash creates empty stash objects from untracked noise files, no cleanup #19 (stash fix, already in PR fix: auto-stash creates empty stash objects from untracked noise (#19) #21)
• Complements: fix: branch conflict — check remote refs, not just local #14 (branch cleanup) and fix: server-side classifier should recognize max_turns_exceeded #15 (max_turns) from prior releases

🤖 Generated with Claude Code