Design the Logseq proxy records so unrelated pull requests don't conflict #210

Since the garden proxy manifest was added to gitpa, nearly every gitpa pull request conflicts in gitp-garden/.logseq-proxy/manifest.json. This was the main objection to the single-manifest design when it was proposed. Redesign the Logseq proxy system so that two pull requests that proxy different pages are unlikely to conflict.

Example

codekiln/gitpa#24 Proxy Microfreak guides, presets and references is conflicting again. Its branch already has a Merge origin/main into codex/18-microfreak-namespace commit, and the conflict is back after one more pull request landed.

codekiln/gitpa#23 Import the public Launchpad namespace with repeatable sync has the same problem. Every gitpa pull request since #12 has changed manifest.json: #12, #15, #17, #21, #22, #23, #24 and #25.

Why it conflicts

• One file holds every proxied page. manifest.json is about 500 lines covering 31 pages and 2 tasks. Any sync rewrites it.
• One page touches four places. Proxying a page adds or changes entries in declarations, imports and pages, and often files too. Two pull requests therefore overlap in several places, not one.
• Sorted keys put new entries next to each other. New sessions sort next to each other (for example GitP/A/Session/26/10/06-… and 26/10/07-…). Git sees two insertions at the same spot as a conflict even when the entries are unrelated.
• Sync rewrites unrelated entries. gitpa#25's sync rewrote the source link and sync date on three proxies whose content hadn't changed. Any other branch that touches those proxies then conflicts too.
• The removal is manual. #205's plan includes hand-deleting keys from declarations, imports and pages for a renamed session. A hand edit to the same file as a sync is another source of conflicts.

Request

Design the proxy records so that changes to different pages land in different files, or on lines that never touch each other. Pick the design after comparing options, for example:

• One small record file per proxied page or task, named from the page the way asset filenames are, so two pages never share a file.
• Storing the proxy record on the proxy page itself as properties, with any index derived at build time and not committed.
• Keeping a committed index only if it can be regenerated, with a merge driver or a regenerate step that resolves conflicts automatically.

Acceptance

• Two branches that each proxy or re-sync a different page merge in either order without a conflict. A test reproduces this with two sibling branches.
• Re-syncing a page writes nothing for pages whose source is unchanged. No rewritten dates or links on unrelated proxies.
• Renaming or removing a proxied page is a sync task operation, not a hand edit.
• gitpa's existing manifest migrates to the new layout, and the RSS build, site checks and proxy docs on Logseq/Entity/Proxy/Page are updated to match.
• gitpa#23 and gitpa#24 can be rebased onto the new layout without hand-resolving manifest conflicts.