Adds the core runtime behavior for active goals on top of the model
tools from PR 3.
## Why
A long-running goal should be a core runtime concern, not something
every client has to implement. Core owns the turn lifecycle, tool
completion boundaries, interruptions, resume behavior, and token usage,
so it is the right place to account progress, enforce budgets, and
decide when to continue work.
## What changed
- Centralized goal lifecycle side effects behind
`Session::goal_runtime_apply(GoalRuntimeEvent::...)`.
- Starts goal continuation turns only when the session is idle; pending
user input and mailbox work take priority.
- Accounts token and wall-clock usage at turn, tool, mutation,
interrupt, and resume boundaries; `get_thread_goal` remains read-only.
- Preserves sub-second wall-clock remainder across accounting boundaries
so long-running goals do not drift downward over time.
- Treats token budget exhaustion as a soft stop by marking the goal
`budget_limited` and injecting wrap-up steering instead of aborting the
active turn.
- Suppresses budget steering when `update_goal` marks a goal complete.
- Pauses active goals on interrupt and auto-reactivates paused goals
when a thread resumes outside plan mode.
- Suppresses repeated automatic continuation when a continuation turn
makes no tool calls.
- Added continuation and budget-limit prompt templates.
## Verification
- Added focused core coverage for continuation scheduling, accounting
boundaries, budget-limit steering, completion accounting, interrupt
pause behavior, resume auto-activation, and wall-clock remainder
accounting.
Adds the model-facing goal tools on top of the app-server API from PR 2.
## Why
Once goals are persisted and exposed to clients, the model needs a
small, constrained tool surface for goal workflows. The tool contract
should let the model inspect goals, create them only when explicitly
requested, and mark them complete without giving it broad control over
user/runtime-owned state.
## What changed
- Added `get_goal`, `create_goal`, and `update_goal` tool specs behind
the `goals` feature flag.
- Added core goal tool handlers that validate objectives and token
budgets before mutating persisted state.
- Constrained `create_goal` to create only when no goal exists, with
optional `token_budget` only when a budget is explicitly provided.
- Tightened the `create_goal` instructions so the model does not infer
goals from ordinary task requests.
- Constrained `update_goal` to expose only goal completion; pause,
resume, clear, and budget-limited transitions remain user- or
runtime-controlled.
- Registered the goal tools in the tool registry and kept them out of
review contexts where they should not appear.
## Verification
- Added tool-registry coverage for feature gating and tool availability.
- Added core session tests for create/get/update behavior, duplicate
goal rejection, budget validation, and completion-only updates.
Eric Traut