fix(ids): skip unfilled placeholders in extractPurpose priority chain

Follow-up to PR #748. With body-level matching gone, the priority
chain still picked up garbage purposes from files that have literal
unfilled template placeholders or JS interpolation in the SOURCES
the chain reads: YAML frontmatter, `## Purpose` sections, `# Title`
headings. The extractor was technically working — the source data
was just full of `{Brief description...}`, `{{TASK_TITLE}}`, and
`${context.componentName}` strings that nobody ever filled in.

## What this PR does

Adds `looksLikePlaceholder(candidate)` that detects:

1. Whole-string single placeholder: `{Brief...}`, `{{var}}`, `${ctx.x}`
2. Leading-token placeholder: `*${name}foo`, `${ctx.x} bar`,
   `{Title} extra`
3. Dominant interpolation (>30% of string): catches
   `${icon} @${id} — ${name}${archetype !== 'Specialist' ? ` (${archetype})` : ''} | ${title}`
   where most of the string is `${}` blocks

The detector is conservative on real prose:
- `Manage feature flags with { enabled: true } syntax` → keeps
- `Use this skill when... ${variable not present}` (<30% interpolation) → keeps
- Empty / whitespace / nullish → keeps (caller handles)

Wired into all four `extractPurpose` strategies (frontmatter,
`## Purpose`, `## Overview`, `# Title`). When a strategy produces a
placeholder, it falls through to the next strategy. When all four
produce placeholders, the function returns the generic
`Entity at <path>` fallback rather than a nonsense purpose.

## Validation

Counted placeholders in `.aiox-core/data/entity-registry.yaml` before
and after this PR:

```
$ grep -E "purpose: ['\"]?\{[A-Z]|purpose:.*\{\{|purpose:.*\\\$\\{" \
    .aiox-core/data/entity-registry.yaml | wc -l
# Before: 16
# After:  0
```

Examples of entries that now have sensible purposes (or sensible
generic fallbacks) instead of unfilled templates:

- `*${taskName.replace(/-/g, '-')}` → now resolves via header or
  falls to `Entity at <path>`
- `{Brief description of what this task does and when to use it}` → now
  falls through to the next strategy
- `Generated: ${new Date().toISOString()}` → captured by dominant-
  interpolation heuristic and skipped
- `${icon} @${id} — ${name}${archetype !== ...}` → captured by
  dominant-interpolation heuristic and skipped

## Tests

12 new tests added to `tests/core/ids/populate-entity-registry.test.js`:

- 4 cover `extractPurpose` falling through placeholders at each
  strategy boundary
- 8 cover `looksLikePlaceholder` directly (positive cases for each
  pattern + negative cases for normal prose that should NOT trip it)

Suite: 91/91 pass (was 79/79 on PR #748).

## Why not just fix the source files?

The placeholders live in templates and generators — they're SUPPOSED
to be unfilled when stored in the framework. They get filled at use-
time when the template instantiates an output. The registry's
inference is reading the template body as if it were a finished doc,
which is the actual bug. Fixing the inference (this PR) is correct;
trying to fill the templates would break their template-ness.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: rafaelscosta