Complete account and tag names in the rule builder

Both fields are free text, and a slug or tag that is slightly wrong produces
a rule that silently catches nothing — or a second, near-identical tag. They
now complete against what already exists: accounts from the folders on disk
and the index, tags from every tag in use plus any named in rules.toml, so a
tag is completable from the moment a rule mentions it.

The completion is ghosted after the cursor and never committed until it is
accepted, so inventing a new tag still works. tab takes it and moves focus
only when there is nothing left to complete; ctrl+n/ctrl+p cycle an ambiguous
prefix, and the hint under the box says how many candidates are left.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -88,19 +88,36 @@ a `▸` against each one the glob currently matches and a running
 "*n* of *m* descriptions match" count.
 
 ```
-▸ glob                            Untagged description              N
+  glob                            Untagged description              N
 ╭────────────────────────────╮      ACME PAYROLL JAN                1
 │ *LIDL*                     │      BOLT RIDE                       1
 ╰────────────────────────────╯   ▸  LIDL SOFIA 4412                 2
   vs. the description            ▸  LIDL VARNA 9911                 1
                                     ZARA MLADOST                    1
+▸ tag
+╭────────────────────────────╮
+│ groceries                  │
+╰────────────────────────────╯
+  tab completes · ctrl+n: 1 more
+
   2 of 7 descriptions match
 ```
 
+Only `gro` was typed in the tag field; `ceries` is the ghosted completion.
+
 `tab` / `↑↓` move between the glob, account and tag fields, `pgup` / `pgdn`
 scroll the list, and `enter` appends the rule to `rules.toml` and retags
 immediately, so the rows it caught disappear from the list. `esc` goes back.
 
+The account and tag fields complete as you type: the rest of the match is
+ghosted in grey after the cursor, and `tab` (or `→` at the end of the line)
+takes it. When several candidates share the prefix, the hint under the box says
+how many, and `ctrl+n` / `ctrl+p` cycle through them. Nothing is committed until
+you accept it, so a new tag is still just typed out in full. Accounts come from
+the folders on disk and the index; tags from every tag in use plus any named in
+`rules.toml`, so a tag is completable from the moment a rule mentions it —
+which is what stops `groceries` acquiring a `grocery` twin.
+
 Leaving the account blank applies the rule everywhere; filling it in also
 narrows the preview to that account. Rules are appended, so anything already in
 `rules.toml` keeps precedence — the preview accounts for that automatically,