feat: move max-runs constraint from engine-specific flags to AWF config

[Copilot]

New Feature

engine.max-continuations previously only worked for Copilot (via --autopilot --max-autopilot-continues N). This moves multi-run orchestration into the AWF config layer as container.maxRuns, making it engine-agnostic.

Implementation details

• New version gate (AWFMaxRunsMinVersion = "v0.26.0"): container.maxRuns is only emitted when AWF supports it; older AWF versions fall back to the Copilot-specific CLI flag as before
• AWF config schema + struct: Added container.maxRuns (integer, min: 1) to awf-config.schema.json and AWFContainerConfig
• awfSupportsMaxRuns(): New version gate helper in awf_helpers.go, consistent with awfSupportsCliProxy, awfSupportsExcludeEnv, etc.
• Validation relaxed: validateMaxContinuationsSupport no longer requires the Copilot engine when AWF >= AWFMaxRunsMinVersion, since AWF handles orchestration engine-agnostically
• Spec doc: specs/awf-container-max-runs-spec.md describes what gh-aw-firewall must implement (GitHub issue creation was blocked by token permissions)

Behavior

AWF version	Behavior
< v0.26.0 (current)	container.maxRuns not emitted; Copilot fallback unchanged
≥ v0.26.0 (future)	container.maxRuns emitted; AWF orchestrates re-runs for any engine

No user-visible change until gh-aw-firewall ships container.maxRuns. At that point, update AWFMaxRunsMinVersion to the released version.