Memory layers: budgeted hot context (#27)

Reginald's context is tiered so the model always sees what matters without ever
copying ticket data into the prompt:
- HOT (this module) — charter + active directives + the focus snapshot, packed
  under a hard token budget (2k). Assembled fresh each turn as the prompt seed.
- WARM — the append-only directive/event ledger + digest, summarized on demand.
- COLD — gitea + sidecar via query_project. Ticket bodies/comments/detail live
  here and are NEVER inlined; the model fetches them by number when needed.

- `assembleHotContext(inputs, budget=2000)`: focus (tiny, always kept) → most
  recent active directives (each while they fit ~⅔) → charter fills the true
  remainder, truncated on a line boundary. Measures the fixed tail exactly and
  reserves for header/joiner/ellipsis so the total never exceeds budget.
- `estimateTokens` (tokenizer-free ~4 chars/token, slight over-estimate so a real
  tokenizer stays under), `activeDirectives` (accepted/amended, most-recent-first).

Acceptance met: hot assembles under the 2k budget even with a ~34k-token charter;
nothing ticket-shaped is inlined (only numbers + titles for focus). +5 core tests;
full core suite + typecheck green.

Follow-up: wire assembleHotContext into the live system prompt in main (needs
charter + directives + focus at chat time) — the assembly + budget is the tested core.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/core/src/index.ts

@@ -109,6 +109,8 @@ export {
 } from './agent/agent-tools.js'
 export { buildProjectView } from './agent/query-project.js'
 export type { ProjectSnapshot, ProjectView, QueryFilters } from './agent/query-project.js'
+export { activeDirectives, assembleHotContext, estimateTokens, HOT_CONTEXT_BUDGET_TOKENS } from './agent/memory-v0.js'
+export type { HotContextInputs } from './agent/memory-v0.js'
 export { CAPTURE_SYSTEM, captureWork, parseCaptureArgs, PROPOSE_ISSUES_TOOL } from './agent/capture-work.js'
 export type { CaptureProposal, ProposedIssue } from './agent/capture-work.js'