fix(key-files): exclude prose docs and project-meta files from candidates

README.md, CONTRIBUTING.md, CHANGELOG, LICENSE, lockfiles, and other
documentation/meta files were flowing into the key-file candidate pool.
Heavily read by humans, but rarely useful as repeated-reference
orientation context — they crowd out actual source files in the token
budget, and prose docs are already surfaced through the docs-injection
path. README.md was the canonical case reported by the user.

Filtered at the candidate-collection layer (read-history.ts) so the
LLM never sees them as options, AND added a belt-and-suspenders rule
to buildV6KeyFilesPrompt instructing the LLM to skip prose docs even
if they slip through. Existing committed project_key_files rows that
contain docs will be replaced on the next Dreamer key-files pass.

Matched basenames are case-insensitive and cover both extension-based
(*.md, *.mdx, *.rst, *.txt) and bare-name forms (LICENSE, COPYING,
NOTICE, README, CONTRIBUTING, etc.) plus the major lockfile families.