docs: spell out the clipped-read answers

web: the detail pane says when its read was clipped

The board has always carried a truncation banner; the detail pane it
shares a template with carried nothing, so an issue assembled from
tables clipped at 2000 rows looked complete, and an id in the tail of a
big tracker was answered "Issue not found." — which the page cannot
know.

Two lines, both in the banner's idiom. A pane built from a clipped read
says so and names the tracker's true size when the issues table itself
was cut; a miss over such a read says the id was not among the rows
read, which is not the same as saying it does not exist. A complete read
keeps the plain "Issue not found.", and the board's own banner is
untouched.

mcpsrv: report the projection's clip on get_issue and list_milestones

Max clips every table read at 2000 rows. list_issues has always said so
(table_truncated, table_total); the other two answers computed over the
same read said nothing, so a rollup over a prefix of the tracker read as
arithmetic about the whole of it, and an id in the tail of a big tracker
was answered "no such issue" — a claim neither read can support.

get_issue now carries the two fields on every answer and splits the
miss: a complete read keeps the plain refusal, a clipped one says the id
was not among the rows read, names the cap and the true total, and ships
the payload (issue: null beside table_truncated) so the two misses are
one field apart rather than one adjective apart. list_milestones carries
the same two fields.

beads: report a clipped read from the remaining projections

docs: read_rows answers strings or null

beads: stop flattening a stored NULL string

mcpsrv: answer a real NULL as null

web: stop printing store paths to the owner

web: stop printing browse errors to the reader

The overview rendered the browse layer's own error text into the page, under
"Could not read history: " — dolt internals and the store's path on our disk,
which nothing else on this surface discloses and which a reader can do nothing
with.

The page now carries a fixed sentence and the reason goes to the log with the
database id (slog + scribe.Err). The view field is a bool rather than a message,
so no error string can reach the template by being assigned to it later. The
empty-database state the page already reported ("No commits.") is untouched:
that is a fact about the database, not a failure.

web: tell a database outage from a missing database

loadRepoForBrowse turned any error from GetRepoByOwnerAndName into a 404, so a
metadata store that could not answer told every reader on the instance that
their database does not exist — for as long as Postgres was down, in the voice
reserved for "there is nothing here". loadRepoForAdmin did the same on the
settings page.

Both classify through repoLookupFailed now: db.ErrNotFound keeps the 404, and
it stays the same 404 the visibility rule renders for a PRIVATE database the
caller may not see, so a masked database and a missing one remain
indistinguishable. Anything else is a 500 through the shared error page, with
the cause logged (slog + scribe.Err) and never rendered.

beads: report a clipped read from every projection

mcpsrv: name the cursor a commit-log miss was asked about

browse: say which cells are actually NULL

mcpsrv: a bad commit-log cursor is a miss, not a fault

web: link issue ids to the tracker that owns them

mcpsrv: answer what is ready across every tracker

browse: report an unparseable start hash as a missing ref

remoteapi: gofmt the integration test

doltsrht: serve /mcp on the web listener

mcpsrv: list the memories a tracker holds