fix: duplicate goal tool registrations, dead RetryPolicy, and stale config docs

[serkanalgur]

Closes #86, #87, #89. Closes #85 (already fixed by 07460d5).

#85 — bare-id model autocomplete

Already resolved in 07460d5: src/orchestrator.ts matches via
tryParseModelRef(m)?.id instead of split('/')[1], so multi-slash refs
no longer throw. No code change here.

#86 — docs/API.md claimed four config blocks; the code writes eight

getSaveableConfig() (src/config.ts:1429-1484) writes eight blocks:
models, budget, selfHealing, dashboard, notifications, gitFlow, effort,
customRoles. The doc said "all four".

The adjacent "not in this schema" list was also wrong in the other
direction: memory, security and communication are not on
NexusConfig at all (deliberately removed — see the doc comments in
src/types.ts), while agents was missing. Corrected to
agents, learning, cost.

Docs-only. No config-writing behaviour touched.

#87 — three goal tools registered twice

goal.status, goal.complete and goal.list were each added to the
nexus namespace twice. The registry keys on namespace_name, so the
second registration overwrote the first and the first set was
unreachable. Removed 38 lines of dead code (51 → 48 registrations, 48
unique, exact 1:1 with the README tool table).

The surviving set is the richer one: it returns autoContinue,
shouldContinue() and createdAt from goal.status, and goal.complete
is the one that persists state. The removed set would have silently
dropped the write had the order been reversed, so the two were compared
field by field before deleting.

Note: the key this writes (nexus-goal) is written but never read —
GoalManager is an in-memory Map. That is pre-existing and out of
scope here, but it means a goal that is never completed does not survive
a reload. Worth a separate issue.

#89 — dead RetryPolicy and an unreachable 'medium' arm

RetryPolicy and Task.retryPolicy were declared but never read or
written anywhere — the real backoff is selfHealing.retryDelay
(src/orchestrator.ts:1123, 1231, 2049, 3379), which already carries
backoffMultiplier. Removed the duplicate concept.

analyzeComplexity only ever produced 'low' | 'high', so the
riskLevel === 'medium' ? 15 : 0 arm at src/orchestrator.ts:3934
could not run. Narrowed the local type and removed the arm.

ComplexityScore.riskLevel stays 'low' | 'medium' | 'high' — that is
public shape and src/templates.ts:65 still produces 'medium', which
test/templates.test.ts:121 asserts. It simply never reaches the
difficulty ladder, because selectQualifiedModel recomputes via
analyzeComplexity instead of reading task.complexity.

Also updated the stale retryPolicy reference in TECHNICAL_DESIGN.md
and three comments in test/config-knobs.test.ts that justified
themselves by pointing at the deleted type.

Tests

test/issue-regressions.test.ts, 14 tests, each verified by mutation
(reverting the corresponding fix makes the right tests fail):

• no duplicate tool names, and exact 1:1 with the README tool table
• getSaveableConfig() block count matches what API.md documents, and
  the "not in this schema" list matches NexusConfig in both directions
• retryPolicy appears in no source file and no doc
• analyzeComplexity exposes no third risk level

Typecheck, lint and build are clean. bun test goes 1322 → 1335 passing,
0 failing.

Review notes

Two things a reviewer may want to weigh in on:

1. The "second registration wins" reasoning for bug: goal.status, goal.complete and goal.list are each registered twice; the second silently shadows the first #87 relies on OpenCode
   SDK editor.add behaviour, which is not verifiable from this repo (the
   plugin package is not vendored). The conclusion is safe either way —
   the set that survives is the richer one, so removing the other is an
   improvement under both readings.
2. Task.retryPolicy is a type-level breaking change for any external
   caller constructing a Task literal with that field. Nothing in this
   repo did, and the field was never read.