Declarative startup.json-driven af up with best-effort subsystem bootstrap

[stempeck]

Summary

• Add .agentfactory/startup.json config that drives bare af up: agent subset selection (null=all, []=none, list=subset), quality/fidelity gate application, start_dispatch auto-start, and watchdog_agents scoping — positional af up <names> bypasses all of it (C-4 backward compat)
• Make worktree-creation failures best-effort (warn + skip + continue) instead of fatally aborting the entire af up
• Tear down the dispatcher session on bare af down (symmetric with startup)
• Centralize escalation target in escalation.go (escalationTarget const + escalationTargets()) replacing hardcoded "supervisor" strings in watchdog and compact_handoff
• Fix git exclude pattern from .agentfactory/ (directory-form) to .agentfactory/* (contents-form) so committed .gitignore re-includes for agent definitions work
• Add --agents flag to watchdog, extract launchDispatchSession() and startDispatch() lenient path, extract applyFidelityGate()/applyGate() for af-up reuse
• Emit pre-flight warning when start set exceeds max_worktrees, and omission warning when a startup subset drops a known mail/notify/escalation sink
• Scaffold startup.json in af install --init with opinionated defaults

Fixes #57

Test plan

• TestLoadStartupConfig_* (7 cases): absent file defaults, bad gate value, full round-trip, empty agents slice, empty object, malformed JSON, scaffold loads
• TestRunUp_NoStartupConfig_AllStart_BareWatchdog: no startup.json starts all agents with bare watchdog
• TestRunUp_ConfiguredSubset_GateDispatchScope: subset + quality gate + dispatch + watchdog scope all apply
• TestRunUp_PositionalArgsWin: af up <names> ignores startup.json
• TestRunUp_EmptyAgents_LoudNotice: agents:[] starts zero agents with loud notice
• TestRunUp_SubsetExceedsMaxWorktrees_Warns: pre-flight warning before start loop
• TestRunUp_OmitsSupervisorEscalationTarget_Warns: omission warning driven by escalationTargets()
• TestRunUp_FidelityOffFromSubdir_GuardChecksRoot: active-formula guard checks resolved root, not cwd
• TestRunUp_FidelityOffNoActiveFormula_Applies: fidelity off applies when no formula active
• TestRunUp_DispatchLaunchFailure_WarnsAndFailsExit: dispatch failure is best-effort (warn + allOK=false)
• TestRunUp_WatchdogAgentsUnknownEntry_Warns: unknown watchdog_agents entry warns at af-up time
• TestRunUp_WorktreeCapHit_SkipsAndContinues: worktree failure warn+skip+continue (not abort)
• TestStartDispatch_* (8 cases): already-running benign, absent config skip, empty default skip, malformed JSON warns, invalid type warns, IO error warns, friendly skip pinned, valid config launches
• TestDown_StopsDispatchSession / TestDown_NoDispatchSession_Silent / TestDown_SingleAgent_LeavesDispatch: dispatch teardown symmetry
• TestApplyFidelityGate_* / TestApplyGate_* (7 cases): gate extraction and guard behavior
• TestWatchdog_PollScopeFiltersAgents / TestBuildWatchdogScope: multi-agent scope filtering
• TestEnsureGitExclude_DoesNotDefeatGitignoreReincludes: live git check-ignore verification
• TestEscalationTargets_IncludesSupervisor: centralized escalation constant sanity

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