engine(repo-map): `renderRepoMap`'s truncation marker is appended outside its own char budget

[JSONbored]

⚠️ Definition of Done: this issue must be completed in full, in a single PR. Do not split this
work across multiple PRs, and do not defer any Deliverable below to a follow-up issue. A PR that
satisfies only some of the Deliverables, stubs a required test, or leaves a checkbox
partially-done does NOT resolve this issue and will be closed.

Context

renderRepoMap in packages/loopover-engine/src/miner/repo-map.ts is documented as rendering "a
bounded plain-text outline" that "Stops once maxOutputChars would be exceeded and appends a
truncation marker, so a caller/prompt-builder can tell the map is partial rather than complete."

Its internal pushLine correctly refuses any line that would push length past maxOutputChars and
sets truncated = true. But the marker itself is appended unconditionally and unbudgeted:

if (truncated) lines.push("… (repo map truncated to fit the output budget)");
return lines.join("\n");

lines.push bypasses pushLine entirely, so the returned string is up to 47 characters (the marker)
plus one newline over maxOutputChars. In the degenerate case where the very first line already
exceeds the budget, the function returns only the marker — a string longer than the budget with no
content in it at all.

The consumer is the prompt packet: this module exists "without paying the token cost of dumping full
file contents into a prompt" (module header), so maxOutputChars is a token-budget guarantee a
prompt builder relies on. A guarantee that is silently violated by a fixed overshoot is worse than one
that is honest about its bound.

Requirements

• renderRepoMap's return value MUST satisfy result.length <= maxOutputChars for every input and
  every maxOutputChars >= 0.
• The truncation marker MUST still be present whenever content was dropped. To make room, the render
  loop MUST reserve the marker's own length (plus its joining newline) from the budget up front, so the
  marker always fits.
• When maxOutputChars is smaller than the marker itself, renderRepoMap MUST return the empty
  string rather than an over-budget marker.
• When nothing is truncated, the output MUST be byte-for-byte identical to today's — no reserved
  headroom may shrink a complete render. (Reserve the marker only against the truncation decision, and
  emit the full budget's worth of content when everything fits.)
• The marker string, the per-entry line formats ("<path>: (skipped: <reason>)",
  "<path>: (no symbols)", "<path>:", " <kind> <name> (line N): <signature>"), the entry order,
  and the default maxOutputChars = 20_000 MUST be unchanged.

⚠️ Required pattern: keep the existing pushLine closure and the outer: labelled loop
(packages/loopover-engine/src/miner/repo-map.ts:344-383); the fix is a budget adjustment inside
that structure. What does NOT satisfy this issue: dropping the marker to stay in budget; truncating
the marker itself; rewriting the renderer as a string-builder or a reduce; or changing the default
maxOutputChars.

Deliverables

• A new named regression test in packages/loopover-engine/test/ asserting that for an entry set
  large enough to truncate, renderRepoMap(entries, 120).length <= 120 and the result ends
  with "… (repo map truncated to fit the output budget)".
• A new test asserting renderRepoMap(entries, 10) === "" when the budget is smaller than the
  marker.
• A new test asserting that a small entry set that fits comfortably renders byte-for-byte the same
  string as before the change (no marker, no dropped lines), at both the default budget and an
  explicit generous budget.
• A property-style test over at least three budgets (e.g. 0, 60, 500) asserting
  renderRepoMap(entries, n).length <= n for each.
• Existing renderRepoMap assertions in the repo-map suite still pass unmodified.

All Deliverables above are required in a single PR. A PR that satisfies only some of them — for example
reserving headroom but not handling the budget-smaller-than-the-marker case, which still returns an
over-budget string — does not resolve this issue.

Test Coverage Requirements

packages/loopover-engine/src/**/*.ts is inside coverage.include in vitest.config.ts and
carries its own engine Codecov flag; the 99%+ branch-counted codecov/patch gate applies here in
full. Both arms of every changed conditional need a test: truncated vs not truncated, budget larger
than the marker vs smaller than it, and the skipped / no-symbols / with-symbols entry arms each
hitting the budget boundary. The length <= maxOutputChars assertion under truncation is the required
named regression test.

Expected Outcome

renderRepoMap(entries, n).length <= n holds for every input, so a prompt builder's token budget is
actually respected instead of being overshot by the marker that claims the budget was respected.

Links & Resources

• packages/loopover-engine/src/miner/repo-map.ts:341-383
• packages/loopover-engine/test/ (the repo-map suite)

[linear-code]

JSO-6691