fleet(github): WS-11 GitHub issues absorber for fleet intake

[Esity]

Summary

• Implements Absorbers::Issues — normalizes GitHub issue webhook events to fleet work items with bot detection, fleet-label dedup, action filtering, and byte-safe description truncation
• Implements Absorbers::IssuesActor — subscription actor with pattern 'github.issues.*' that delegates to the Issues absorber
• Implements Absorbers::WebhookSetup mixin — idempotent webhook registration + fleet label creation (fleet:received, fleet:implementing, fleet:pr-open, fleet:escalated)
• Implements Absorbers::Helpers — shared utilities (bot detection, label check, fingerprinting, transport_connected?)
• Wires all absorber modules into the extension entry point

Design adherence (from 12-github-absorber.md)

• No set_nx — assessor is the single dedup authority; absorber only does source-specific dedup (bot filter, label check, action filter)
• Payload in Redis — raw GitHub payload stored at fleet:payload:{work_item_id} (24h TTL); AMQP message carries only the reference key
• publish_to_assessor — extracts task_id from Runner::Status.generate_task_id Hash, sets master_id: task_id, function: 'assess' (String)
• instructions: [] and context: [] populated on every work item
• Description truncation — body.bytesize > max_bytes ? body.byteslice(0, max_bytes).scrub('') with runtime fallback to 32768
• Publish failures propagated — { absorbed: false, reason: :publish_failed } returned, not swallowed

Test plan

• 61 specs, 0 failures (bundle exec rspec spec/absorbers/)
• 0 rubocop offenses (bundle exec rubocop lib/legion/extensions/github/absorbers/ spec/absorbers/)
• Absorbers::Issues.absorb returns absorbed: false for bots, already-claimed, ignored actions
• Description > 32KB is truncated to byte boundary
• WebhookSetup#setup_fleet_webhook is idempotent (returns existing: true if webhook exists)
• WebhookSetup#setup_fleet_webhook returns success: false when create_webhook returns no id

Depends on

• WS-02 (runner prerequisites) — already merged
• WS-06 (lex-assessor queue) — assessor must be running for publish_to_assessor to deliver; absorber skips silently in lite mode

Files changed

lib/legion/extensions/github.rb                         (wired)
lib/legion/extensions/github/absorbers/helpers.rb       (new)
lib/legion/extensions/github/absorbers/issues.rb        (new)
lib/legion/extensions/github/absorbers/actor.rb         (new)
lib/legion/extensions/github/absorbers/webhook_setup.rb (new)
spec/absorbers/helpers_spec.rb                          (new)
spec/absorbers/issues_spec.rb                           (new)
spec/absorbers/actor_spec.rb                            (new)
spec/absorbers/webhook_setup_spec.rb                    (new)