Wiki never indexes context/stack.md, project-specific context files, or patterns added after setup

[theyashasvipandey]

Some knowledge files in .mex/context/ and .mex/patterns/ never become Wiki entities, and
nothing reports it. Their content and groundings are unreachable through wiki query,
wiki for-code and the Hub's Context view.

1. context/stack.md and project-specific context files are always skipped

wiki migrate classifies context files through a fixed table, CONTEXT_ROLES
(src/wiki/migration/classify.ts:101). The table covers architecture, conventions, setup,
risks and decisions. context/stack.md is not in it, even though setup writes it for
every project. Neither are the domain files that population creates (routing.md,
payouts.md, …). Each one gets:

No classification rule determines the type of context/stack.md. … Migration leaves this file unchanged.

Measured on two scaffolds that setup populated:

scaffold	context files skipped	groundings in skipped files
Hono	routing.md, rpc-client.md, stack.md	8
project set up with 0.8.2	stack.md + 3 domain files	17

On Hono, 19% of the questions in an evaluation set could not be answered from the Wiki at all
because their answer lives in one of these files.

2. Patterns added after setup are never adopted

On mex itself, three pattern files (hub-first-run-onboarding.md,
syntax-extractor-review.md, usage-telemetry.md) have no entity. wiki migrate --dry-run
shows migration would adopt all three. Nothing re-runs migration after setup, and neither
mex check nor wiki validate says a knowledge file is outside the Wiki.

Expected behavior

• Add a rule for context/stack.md (it is a list of technologies; guide, or a file-level
  entity with a new stack type, are both plausible).
• Give project-specific context files a default rather than an abstain. For example, adopt a
  file-level entity typed from frontmatter, or fall back to a generic type, so population output
  is always indexed.
• Report un-adopted knowledge files as a check or wiki validate diagnostic, with
  wiki migrate as the remediation.