Nests: team rooms with core crows and borrowing

- Server: nests API, persistence, core-first picking, borrowed flag
  (built by the backend agent against EVENTS.md; tests pass with -race)
- UI: nests in the sidebar, one feed per nest, header shows the core
  team, New nest dialog, borrowed crows labeled, per-nest waiting count
- start.command: one-click local server in controlled mode (mock crows,
  localhost only, state in .crows/)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UQnSw4RRWp8uqpj3nsnnXF

flock.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"slices"
 	"strings"
 	"sync"
 	"time"
@@ -82,22 +83,46 @@ type Flock struct {
 	log   *FlightLog
 	sem   chan struct{}
 
-	mu   sync.Mutex
-	next int
+	mu     sync.Mutex
+	next   int            // rotation through the whole flock (the todo nest)
+	core   map[string]int // per nest: rotation within its core
+	borrow map[string]int // per nest: rotation through crows outside its core
 }
 
 func NewFlock(brain Brain, g Guardrails, log *FlightLog) *Flock {
-	return &Flock{brain: brain, g: g, log: log, sem: make(chan struct{}, g.MaxCrowsInFlight)}
+	return &Flock{
+		brain: brain, g: g, log: log, sem: make(chan struct{}, g.MaxCrowsInFlight),
+		core: map[string]int{}, borrow: map[string]int{},
+	}
 }
 
-// pick rotates through the roster so every crow gets a chance to build a record.
-func (f *Flock) pick(n int) []string {
+// pick chooses n crows for a job in a nest. A nest with no core (todo) rotates
+// through the whole roster so every crow gets a chance to build a record.
+// Otherwise core crows go first (rotating within the core), and any extra
+// crows are borrowed from outside the core (rotating).
+func (f *Flock) pick(nest Nest, n int) []string {
 	f.mu.Lock()
 	defer f.mu.Unlock()
-	out := make([]string, n)
-	for i := range out {
-		out[i] = roster[f.next%len(roster)]
-		f.next++
+	out := make([]string, 0, n)
+	if len(nest.Crows) == 0 {
+		for range n {
+			out = append(out, roster[f.next%len(roster)])
+			f.next++
+		}
+		return out
+	}
+	core := nest.Crows
+	for range min(n, len(core)) {
+		out = append(out, core[f.core[nest.ID]%len(core)])
+		f.core[nest.ID]++
+	}
+	outside := slices.DeleteFunc(slices.Clone(roster), func(c string) bool { return slices.Contains(core, c) })
+	if len(outside) == 0 {
+		outside = core // the whole flock is the core: nobody left to borrow
+	}
+	for len(out) < n {
+		out = append(out, outside[f.borrow[nest.ID]%len(outside)])
+		f.borrow[nest.ID]++
 	}
 	return out
 }
@@ -169,11 +194,11 @@ func (f *Flock) Triage(ctx context.Context, w Watcher, item string, budgets ...*
 	return t, nil
 }
 
-func (f *Flock) Execute(ctx context.Context, w Watcher, item string, t Triage, budgets ...*Budget) (Outcome, error) {
+func (f *Flock) Execute(ctx context.Context, w Watcher, item string, t Triage, nest Nest, budgets ...*Budget) (Outcome, error) {
 	switch t.Mode {
 	case "split":
 		// Split depth is 1: subtasks go straight to crows, never back to the Sentry.
-		crows := f.pick(len(t.Subtasks))
+		crows := f.pick(nest, len(t.Subtasks))
 		reports, err := f.fanOut(ctx, w, crows, t.Subtasks, budgets...)
 		if err != nil {
 			return Outcome{}, err
@@ -181,7 +206,7 @@ func (f *Flock) Execute(ctx context.Context, w Watcher, item string, t Triage, b
 		return f.reconcile(ctx, w, item, crows, t.Subtasks, reports, budgets...)
 	case "murder":
 		n := min(f.g.MurderSize, f.g.MaxCrowsInFlight)
-		crows := f.pick(n)
+		crows := f.pick(nest, n)
 		tasks := make([]string, n)
 		for i := range tasks {
 			tasks[i] = item
@@ -192,7 +217,7 @@ func (f *Flock) Execute(ctx context.Context, w Watcher, item string, t Triage, b
 		}
 		return f.reconcile(ctx, w, item, crows, tasks, reports, budgets...)
 	default:
-		c := f.pick(1)[0]
+		c := f.pick(nest, 1)[0]
 		ans, err := f.ask(ctx, w, c, crowSystem, "TASK: "+item, budgets...)
 		return Outcome{Answer: ans, Crows: []string{c}}, err
 	}